Is Air Made of Matter?

I was at Chicago's Museum of Science and Industry earlier this week, which was packed with elementary school and middle school kids on field trips! During a chemistry demonstration, the kids were asked what they understood about matter. Now, just about everyone knew a rock was matter and that people consist of matter. Almost none of the kids knew whether or not air was up of matter. It's easy to understand the uncertainty. After all, it's invisible. Here's the answer to the question, plus ways you can prove whether or not air is matter.