Here is a trick that is hard to believe. Fill a plastic sandwich bag with air, either by blowing up the bag or by pulling it quickly through the air with the mouth of the bag
held open.

Hold the air-filled bag tightly over the mouth of a jar and tie it tightly with a string or a rubber band.

Press down on the bag, lean on it, rest a book on it, and describe what happens in each case.

Now remove the bag, empty it of air, and tie it again -- but this time with the bag inside the jar.

When the bag is secure, hold the jar and try to pull the bag out.

What's the matter? Is this a whole classroom full of wimps? Why can't you pull out the bag?

The plastic bag has air resting in it -- the air weighs much more than you can lift.

Does this mean that plastic bag has to stay in the jar forever? How do we get it out of there? Probably, the only way is to untie it, and let some air go in between the jar and the bag to help us pull out the bag. Let's try that.