Why does a bag of air push back when you squeeze it?

Picture a plastic bag, all puffed up and zipped shut. Squeeze it with your hands, and it feels soft and bouncy. Your hands press in, and the bag presses right back.

It looks like there is nothing inside that bag. So why does a bag of air push back when you squeeze it? Let's find out together.

The bag is not empty at all. It is full of air. You cannot see air, but air is real stuff. It fills every corner of the bag, like water fills a cup.

When you zip the bag shut, it becomes airtight, which means no air can slip in or out. So all the air stays trapped inside. It has nowhere else to go.

Now you squeeze. Your hands push the sides of the bag in, so the space inside gets smaller. The airtight bag keeps all the air in that smaller space. What do you think the air does when it gets squashed into less room?

The air pushes out! Air is lots of tiny bits, much too small to see. With less room, those bits bump the inside of the bag more often.

All that bumping pushes on the bag from the inside. Your hands are on the outside, pressing in. So you feel the air pushing back against your hands.

Squeeze a little harder, and the air gets even less room. Then it pushes back harder too. A gentle squeeze gets a gentle push.

Airtight. Your squeeze gives the air in an airtight bag less room, so it pushes back.

Sleep softly now, and dream of puffy bags full of air.