The bags come with both ends open. Tie one end closed and then ask a member of the class to try to blow up the bag with one breath. If they insert their mouth into the opening of the bag they will not be able to do so. However, if they blow into the bag from a distance of 1 foot from the opening they can easily blow up the bag with one breath.

A cool variation of this is to use a cardboard tube and soap bubble solution. Have the largest students see who can blow the biggest bubble by blowing directly into the tube. Then show that the smallest child can beat this if they hold the tube about 4 inches from their mouth when they blow into the tube.