Why does a drink go up a straw?

When you drink through a straw, it feels as if you’re pulling the drink up with your mouth. But you can’t pull a liquid along behind you. So what really gets it into the straw?

Air pushes

Above us are tens of kilometres of air, and its weight compresses the air near the ground like a spring. Compressed air pushes in all directions: on every square centimetre about as hard as a one-kilogram weight, on a palm about a hundred kilos in all. We don’t feel it, because it pushes on the palm equally from both sides, and the pressure inside the body is the same. But put your palm on a vacuum cleaner nozzle: the air on one side pushes a bit more weakly, and your palm sticks at once.

It pushes on the surface too

Air also pushes on the surface of the drink in the glass, and equally everywhere: around the straw and inside it. That’s why the drink in the straw is at the same level as in the glass.

What your mouth does

When you close your lips round the straw and pull your tongue down from the roof of your mouth, the space in your mouth gets bigger. The air in your mouth and in the straw has more room, spreads out and pushes more weakly: there’s a partial vacuum. Outside, the air keeps pushing on the surface just as hard, so it pushes the drink up the straw.

Air can hold water up too

Dip a straw in water, block the top end with your finger and lift it out. The water stays inside: the air under your finger spreads out a little and pushes more weakly than the air from below. When you lift your finger, the pressure at the top and bottom evens out and the water runs out.

See for yourself

Put two straws in your mouth, one in the drink and the other outside next to the glass. However hard you try, the drink won’t come up. Air from outside flows into your mouth through the second straw, so the pressure in your mouth doesn’t drop, and the air outside doesn’t push the drink up.

A hand pump is a big straw

A hand pump at a well works just like a mouth with a straw. When you push the handle down, a piston rises in the pipe, the space beneath it grows and a partial vacuum forms, so the air pushing on the water in the well pushes it up the pipe. One-way valves then let the water above the piston, and on the next stroke it pours out of the spout.

No higher than 10 metres

Air pushes on every square centimetre about as hard as a one-kilogram weight, and that’s exactly what a column of water 10 metres high weighs. Air can’t push water any higher, even if there were no air left in the straw at all. A hand pump has the same ceiling: it only works for a shallow well, and for a deeper one the pump has to be down by the water.

So why does the drink go up?

You don’t pull the drink up with your mouth. You just lower the air pressure inside it, and the air pushing on the surface in the glass pushes the drink up the straw. Without air around, a straw wouldn’t work.