Students blew up balloons to help illustrate how even air has density, even though we can't see it. By comparing a blown up balloon in a bucket of water to a flat balloon, we noticed that the air inside changed the buoyancy of the balloon, and therefore its density. Similarly, when we placed a blown up balloon in a freezer, we saw it change shape as the air inside cooled down - again changing its density. Who knew studying air could be such a gas?!
Try doing some science experiments like this at home and see what you discover!