This is from Sanjoy Mahajan's The Art of Insight Problem 1.2

Estimate the mass of air in your bedroom.

Pacing out my bedroom it's roughly 3m on each side, and the ceiling is maybe a meter higher than I am, so say 3m high. So the volume of my bedroom is about (3m)³ or 27m³.

The density of air is roughly 1 kg/m³, so the mass of air in my bedroom is around 30 kg.

Interestingly this is about the same mass as a small box of books. This is hard to appreciate because I can't really imaging moving the air in a room.