Lightning is a giant electric spark, and Gareth’s brought his own little bit of lightning into the How studio.

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.