How do you find the density of air?

Good one! Density is usually found by weighing a sample of a material and measuring its volume, but how do you weigh a sample of air? As this is very likely a homework question (preparatory question for a lab?) I'll only give some useful hints.

If you want to weigh your cat on a bathroom scale but can't get her to stand still on it, you can weight yourself, then weigh yourself again while holding the cat; the increase is the cat's weight. If you've weighed yourself with the cat first, you can still weigh yourself without her and then her weight is the decrease. Get it? You can weigh something by removing it and computing the decrease.