Imagine this cylinder of water is a flood of water in the basement of your home. Flood’s aren’t normally dyed red, but this one is.

Here’s the fire hose, pumping water at a furious rate. What use is that for clearing the flood – surely it’ll make everything worse?

Well, no. The fire crew stick another bit of hose into the floodwater…

…and amazingly, the flood recedes.

The fire hose pumps the flood water out of the cylinder (or your basement), carrying it away. You can just see that the spray of water from the host has turned pink here, coloured by the red dye in the floodwater.

What’s going on? Allow Fred to explain the venturi effect, with a simple paint sprayer.

Off he goes, blowing away at the cameraman.

Thanks, Fred. We’ve got the idea.

No, Fred. Really, that’s enough. Fred!

So, blowing through the horizontal tube reduces the pressure at the top of the vertical tube. Air pressure then forces the dye up the vertical tube, and it’s sprayed all over the cameraman. Got it.

The same thing is happening with the fire hose, only using water rather than air. The fast-flowing water reduces the pressure in the pipe dropped into the floodwater. So the floodwater is carried up the pipe, and away. That’s how a fire engine can fight a flood.