...sock Gareth in the gob.

Ooh! But of course, it only looks like Gail hit Gareth from where we -- and Fred -- are sitting.

From another angle, it's obvious that she aimed to miss. Lesson one: you have to agree what you're going to do with the other actors, so they don't accidentally flinch into your punch. Because then you really do hit them in the face. Oops.

As for the sound -- that's easy. Your eye is drawn to the flying fist and Gareth's head, so you don't notice Gail slapping her stomach just as the punch 'connects.'

If you're filming a 'fight' on video, you can also do the classic run-the-tape-backwards trick: start off as if you've just lamped your opponent in the chops, then...

...pull back. Run that backwards and... you know, actually it didn't look very good, did it? Ah, well.

Still, now we know the basics, there's nowhere for a good punch-up that's quite as good as the Wild West.

Meet Maverick McKenna, double-crossing horse-thief.

Meet Calamity Jones, low-down varmint.

But wait! Who's this approaching?

"Dead-Eye" Dinenage, Sheriff of these here parts, and known for his strict enforcing of the local laws: all Post-It notes to be handed in, and absolutely no fist fights.

Too late.

Much too late.

Far, far too late.

Ah, well. hopefully they'll have recovered for the next show -- see you next Friday!

To win the chocolates, all Gareth and Gail have to do to tell Fred how many chocolates he has in his hands. His hands will be open, but their eyes will be closed, and they have to cross their fingers for luck.

Gail first. She thinks she can feel two chocolates. Bzzzt! Wrong!

Next, Gareth. He think the same -- two chocolates. Bzzt!

Fred really is only holding one chocolate in each hand. What's going on?

Normally, when you feel something, it sits between your fingers like this. Your brain correctly works out that there must be only one object, touching the insides of two fingers.

But if you cross your fingers, the object touches the outsides of your fingers. Your brain, not used to this, gets all confused, and usually reckons there must be two objects.

This works with most round or curved objects -- it can even feel weird if you cross your fingers and then stroke your nose with them. Go on, have a go.

As well as going up, they can also reach out sideways. How far? Let's find out...

While the crane's hoisting him out, Gareth takes a moment to explain.

The hoist has a long arm, at one end of which is... well, the crane. That's a massive weight.

Dangling from the other end is Gareth. Who -- joking aside -- doesn't weigh very much compared to the crane.

So he can be a long way from the pivot point before the whole thing...

...overbalances, and dumps him in the drink.

That's the theory. Time to see what actually happens.

Far enough?

Nah, further!

Look at that! The crane's so heavy, it can safely balance Gareth right at the end of this massive overhang. Its computer monitors the loads on the stabilising legs, and makes sure the operator is perfectly safe at all times. Amazing.

Slop-slop goes the paste...

But not so much of the stick-stick goes the paper. How's Gareth doing?

No better, it's just peeling apart. Gail's glue is grot!

Unfortunately, Gail isn't the first to invent a non-stick glue. In 1970 in Minnesota, Spencer Silver was working on a new super-strong glue. Accidentally, he invented a new glue that was completely rubbish -- he stuck it on a shelf, thought better of that idea and placed it on a shelf, and forgot about it.

However, a friend of Spencer's called Arthur Fry sang in the local church choir. Like everyone else, he used bookmarks to note the songs he was going to sing.

But you know what happens to bookmarks -- they fall out, and you lose your place.

Spencer and Arthur had a brilliant idea: the rubbish glue was no good for sticking things together, but it was plenty good enough to hold a bookmark in place.

The gently-stuck bookmarks didn't fall out, but best of all...

...they peeled off without damaging the book!

The result: the Post-it note. Which, of course, is one of the most popular office inventions since the paperclip. Spencer, meanwhile, made millions out of his 'useless' weak glue.

It's an arc welder, a device usually used to melt two bits of metal together. It makes a spark that's so bright, it could damage the cameras. Your computer at home is probably OK, though.

Arc on! The voltage isn't very high -- so the spark isn't very long, by the standards of lightning -- but the current is extremely large. More than enough to kill you, and enough to make lots of light.

When the arc comes into contact with water -- like when Gareth dunks the electrodes into a bowl of the stuff, crazy boy that he is -- the intense heat generated boils the water, turning it into steam.

Bear with us, there's another experiment: inside this tin is a small amount of water, and the lid is on very tightly. As the gas flame boils the water inside the tin, it turns to steam. But steam takes up more space than water, so the pressure inside the tin increases and increases until...

...it blows the lid off.

But what, you might wonder, has any of this to do with blowing your socks off? Well, the wettest parts of your body -- at least, outside -- are your feet. The sweat from your feet gathers in your socks.

Eu. Now, your shoe keeps the moisture contained, rather like the tin did.

So if you're hit by lightning, the intense heat can boil the water in your socks, and the expanding steam can...

...blow your boots off. Socks, too.

The trouble is, if you overload your ship, it could sink. Which tends to make you no money at all, and hence isn't the best business plan.

But there's another problem too. Suppose this is your ship -- yes, we know it looks like a test tube -- with the cargo represented by the ball bearings.

Now, suppose you load your ship up in the icy waters of the North Atlantic, being careful not to fill it so much that it sinks.

Then, you sail your cargo to the warmer waters of the tropics. Perhaps even a river estuary. It's much warmer, so the water is less dense, and the river water dilutes the salty sea water, so it's even less dense.

The result is that your ship floats much lower in the water...

...so low that heavy seas could swamp it, sending it to the bottom. Oops.

So what do you do? Well, you could mark your ship with a safe level. When you load your ship, you stop loading when that mark touches the surface of the water.

...like this.

Then, when you sail into warmer waters, you've more of a margin, and your ship stays safe.

This is exactly what shipping companies do. If you look on the side of a ship in dock, you'll see these markings -- they're called Plimsol lines, after the politician who enforced their use after a spate of tragic accidents.

The different marks have different meanings -- here, the two to the left of the vertical line are the safe loading levels for the tropics (top) and arctic (bottom).

The poison is extremely potent, causing paralysis in only seconds. Just two millionths of a gramme (2 microgrammes) are enough to kill a human.

But this is How, where we know no fear -- so these little fellas are poison arrow frogs. Ooooh! Scary!

Or cute. They're actually quite cute, huh?

When they're stressed -- say, when something tries to eat them -- they secrete a poison from their backs.

In the wild, they eat poisonous beetles -- then they collect and concentrate that poison, and exude it from glands in their backs.

Even a tiny amount of poison on your bare skin could kill you in seconds. So...

...doing this might seem crazy. Prodding a deadly-poisonous frog? No, Gail, what are you thinking?

It's not what you might think. Gail's frogs have been bred in captivity, and fed on these -- baby crickets. As you might have guessed, the frogs themselves aren't poisonous at all -- it's the poison they harvest from the beetles they eat that you have to be careful of.

So, how poisonous is a poison arrow frog? In the wild, it's deadly -- but if it's only eaten crickets, it's perfectly safe.

Creep up behind it and shout 'BOO!'? Perhaps. But Gareth has another plan. He's going to...

...blow at it. Eh?

If you have a little stack of cups, and you blow hard into the gaps between them, you can make the top one pop out of the stack, like this. Which, by itself, is fairly dull.

However, if you have a large stack of cups like this, and an air jet from a compressor:

...you can make the cups really jump!

There they go!

Pointless. But fun, eh? You can try this for yourself, using a can of compressed air from a photography shop. Be warned, though -- you need the larger cans. The smaller ones are just too weedy to do the job, and even with the big ones it takes a bit of fiddling.

As the pea rattles around, it makes that shrill warbling sound we like so much. But what happens if the pea gets stuck?

What if it jams in the opening, like this? That's exactly what happened to a chap called Ron Foxcroft, who was refereeing the Olympic Games basketball final back in 1976.

He saw a rules violation, blew his whistle, and... nothing happened. The pea was stuck, no sound came out, and the players carried on playing. Frustrated, Ron invented:

The pea-less whistle!

Yes, it's perfectly loud enough.

So now, Fred can referee a match -- basketball in this case. Wait -- that's umpiring, isn't it?

No time to argue, Gail's about to...

...commit a foul. Oh, that really is foul. Who needs to see Gareth's knobbly knees? Ref!

Gail may think she's got away with it...

...she may take the shot...

...she may even score...

...but there's no mistaking the shrill call of Fred's whistle. McKenna -- sin-bin for you! Hah!

You can find out more about Ron's whistle from his company, Fox 40. So that's How you can be sure to stop play -- and that's How for now. See you after the World Cup!

Gareth shuts his eyes and heads off.

Not too bad, but...

Ah. He hasn't quite managed to stay on the line. How does Fred do?

Worse, sorry to say. What's going on? Why are they both veering to the right?

When you walk, you tend to lean a little more on one leg than the other -- your dominant leg. If you prefer to kick with one leg, it'll be that one. Leaning like that unbalances your walking gain -- you take longer strides with your weaker leg as you lean on the stronger one.

As a result, you tend to walk in a gentle curve -- walk for long enough, and you'll walk right round in a big circle!

...it always points towards magnetic North, which in the How2 studio is...

...over Gareth's shoulder there.

But here's an ancient Chinese invention, two thousand years old.

OK, so it looks like a plate with a little spoon shape resting on it, but look at what the spoon does...

Here, it's still wobbling around. But if you saw the show, you'll have seen that it came to rest with its handle pointing more-or-less magnetic South. That's because the 'spoon' is made out of lodestone, a rock with an iron content so high, it's magnetic.

The spoon, or ladle, shape looks uncannily like:

...this constellation of stars. Recognise it? In Britain we call it The Plough, but Americans call it 'The Big Dipper' -- not after a fairground ride, but because a ladle, in America, is called a 'dipper.'

Follow this line from the bowl of the dipper, and you reach Polaris -- the pole star, which happens to be magnet North, wherever you are on the Earth.

So the Chinese compass is a fantastic navigational device. Or it would have been, had they used it for that. In fact, they used it 'geomancy' -- lining up their houses with the mystical powers of the land. That is, the sort of thing we'd now call 'feng shui.' But still -- how useful is a compass? For navigation, very!

Here's a mortar cannon. How far it can fire a ball depends on the size of the cannon...

...the size of the cannonballs, how much gunpowder you use, but also...

...the angle at which the cannon is set.

First, the cannon is set to a high angle -- sixty degrees above the horizontal.

Whumpf!

Now pay attention - that's how far the cannonball went.

For the next shot, the cannon is set to an angle of thirty degrees -- a much lower shot.

Whumpf 2!

Perhaps surprisingly, the cannonball splashes at almost exactly the same spot.

How about forty-five degree?

Whumpf 3!

Yes, the cannonball goes noticeably further.

45 degrees is the optimum angle: the ball goes high, but not so high that all the energy from the gunpowder is spent lifting it up rather than flinging it forwards.

It's not just cannonballs, either -- 45 degrees is usually the best angle to launch anything at, to get the best range.

Here's one of them. Scorpions have been around since dinosaurs roamed the Earth. Not Scotty himself here, of course (he's not even as old as Fred) but as as a genera -- a group -- of species.

They're best-known for the sting in their tail -- a nasty spike that delivers a dose of poison. In some species, the venom is so powerful it can kill a human.

The thing is, Scotty here wasn't the only scorpion Gail brought with her. There were more.

Fred's not too happy about this. How are they going to find the others?

Well, scorpions like dark, gloomy areas -- they're nocturnal. So their dark colouring makes them quite hard to spot. even with a torch...

...they're quite hard to pick out. But look what happens under ultraviolet light:

The scorpions fluoresce! The glow brightly against the dark background.

Nobody's quiet sure why they do this. Perhaps it's to attract insects for them to eat, or maybe it's just a consequence of the material they're made from. But nevertheless -- if you want to spot a scorpion, use ultraviolet light. That's How!

There's a little magnet..

...and, slung between two electrical contacts, a coil of insulated copper wire.

Here's the coil. In this design, it's the coil that does the spinning around.

Now, it's a bit hard to explain what the insulation does on the wire, but it's very important, so here's a giant model to help Gareth explain.

The copper wire coil is the big ring in the middle, but we need to pay attention to the ends.

Here, the insulation has been scraped away from half of the wire. So the shiny side, here, is bare metal...

...but the lacquered side is still insulated -- electricity doesn't flow through it.

So, this way up (click the picture if it's too small to be clear), the bare wire makes an electric circuit.

But this way up, the insulation breaks the circuit.

Back to Gareth's motor. It's hard to see where the insulation has been scraped off, but it's just like the model. When the bare metal is touching the upright contacts, the circuit is made, and the wire coil twitches in the magnet field.

But when it's resting on the insulation, the circuit is broken, and it just freewheels.

That on-off-on-off routine is what allows the coil to spin -- half of the time it's pulled toward the magnet, and the other half of the time its momentum carries it further around, until it's next in the right position to be attracted to the magnet.

That's a pretty simple motor, but here's an even simpler one, with even fewer parts. A magnet...

...a steel screw (with a tab of paper so you can see it when it spins, in a moment)...

...a battery...

...and a piece of wire. The magnet clamps on the end of the screw, and it's strong enough to hold the screw (and itself!) dangling from the battery. But there's very little friction from the point of the screw, so the screw and magnet can spin around quite freely.

Touch the wire to the magnet and the circuit is completed. The flow of electricity interacts with the field of the magnet, producing a twisting for (a torque) on the magnet and screw.

The result is that the magnet and screw spin round. Amazing, and quite possibly the simplest form of electric motor.