Why does a suction cup stick to glass?

Maybe you have pressed a suction cup onto a window or a shiny bath tile. Think of the soft rubber squashing flat under your fingers, cool and bendy. Then you let go, and it holds on tight!

There is no glue on that little cup, and no hook either. So why does a suction cup stick to glass? Let's find out together.

Air is all around us, even though we cannot see it. It presses on everything it touches, on your hands, your toys and every window, from every side. That push has a name, air pressure, which means the push of air on everything it touches.

When you press the cup onto the glass, the soft rubber bends and flattens. As it flattens, it pushes the air out from underneath. Now only a little air is left inside the cup.

At the same time, the rubber rim lies flat against the glass. Glass is so smooth that the rim fits it with no gaps at all. Together they make a seal, which means a tight edge that lets no air through.

When you let go, the bendy rubber tries to spring back into its round shape. That makes a bigger space under the cup, but the same little bit of air has to fill it. Spread out thin, that air pushes back only weakly, so the air pressure inside is small. Outside, the air pushes hard, and inside, it pushes softly, so which way do you think the cup gets pushed?

The strong air pressure outside wins! It presses on the whole back of the cup and pushes it against the glass. It works like a big invisible hand holding the cup still.

The cup stays put for as long as the seal holds. No new air can sneak under the rim to fill the cup and push back from inside.

Air pressure. Seal. When you press a suction cup onto smooth glass, you squeeze the air out, the seal keeps new air from getting back in, and the strong air pressure outside pushes the cup against the glass.

Next time you see a suction cup on a window, you can smile and know the air itself is holding it there for you.