Full of Hot Air

Air has mass, volume, and density.  

We compared  two balloons tied to a yard stick.

•  Did they have the same mass; amount of stuff in them?
•  Did they take up the same amount of space; have the same volume?
• If they have the same mass and volume then the balloons have the same density.

Then BOOM! One of the balloons changed the amount of mass and volume it contained. 

http://www.boston.com/news/health/blog/2010/09/_you_cant_make.html?rss_id=Top+Stories