Hot Air Balloons

The other day my two daughters and I were sitting at the kitchen table.  I was reading a book with my back to the kitchen window and my daughters were coloring pictures, facing the window.  Suddenly they both got excited and started pointing and telling me to look at the window.  I turned around and saw this:

A hot air balloon!  I've never been in a hot air balloon, and neither have my daughters.  Part of me thinks it would be cool to ride in a hot air balloon, but then the part of me that is terrified of heights steps forward and slaps me!!!

Seeing this hot air balloon offered a great opportunity to explain to my daughters, especially my 6 year old, how hot air balloons work.  It basically boils down to density.  A burner is used to heat the air inside the balloon.  Hot air is less dense than cooler air, so the the balloon rises.  It's similar to mixing a glass of water with a glass of oil.  The two substances have different densities and separate with the lower density material rising.  

To keep the balloon from falling, you need to keep re-heating the air to keep it less dense than the surrounding air.  If you want to land, simply stop heating the inside air, it will cool and become more dense, thus causing the balloon to slowly sink.  

Physics in action!