What does air weigh?

The girls have come up with some interesting questions lately but the one yesterday morning really threw me.

"if helium balloons float because helium is lighter than air then what does air weigh?"

after much searching on the Internet and loads of explanations about density, temperature and  elevation, we found a great site all about how helium balloons work :)

http://science.howstuffworks.com/helium2.htm

We're off to play with some balloons now.