Breathe In...Breathe Out...
You'll find helpful experiments, videos and worksheets to enhance your child's understanding of their respiratory system. The experiment was one I remembered doing in 5th grade. When I did this at our homeschool co-op, the kids absolutely loved it and wanted extra turns.
Here's how you do it: Fill the gallon jug or 2 liter (just depends what you have laying around). Hold your hand over the opening of the jug and turn it upside down. Place in a sink of water high enough to cover the opening, so no water escapes from the jug. Place a tube in the opening of the jug allowing the other end to be used for blowing. Let you child blow the water out and they can see their lung capacity compared to others.