What’s actually happening when we use a straw, and can stainless steel rust?

We explore the curious questions that science can answer

It’s all to do with air pressure... (Getty/iStockphoto)

When we use a straw to suck water from a glass, what is actually happening?

This is all to do with air pressure. We have to think about the pressure in two places, first the pressure of air inside the straw, and also the air pressure outside, which is pressing down on the surface of the water in the glass. When you suck with your mouth, you remove some of the air from the inside of the straw. This means that the air pressure inside the straw is less than the air pressure outside. This difference in pressure means that air from outside tries to get inside the straw to even out the difference. As the air tries to get in, it pushes the water ahead of it, and hence up the straw.

It sounds odd, but when you suck water through a straw, it is actually being pushed from underneath, rather than being sucked from above.