Spring is a perfect time to observe how trees take in carbon dioxide and let out oxygen. Just try this experiment passed on to us by the Seven Ponds Nature Center in Dryden, Michigan.
On a sunny day, use a piece of string to tie a clear plastic bag around a clump of green leaves on the end of a tree branch. Leave it for an hour or two.
When you come back, the inside of the bag should be coated with drops of water. Why? Instead of a nose and mouth, plants have lots of tiny holes, or stomata, in their leaves through which they breathe. Just like our own breath, plant exhalation is full of moisture, which condenses on the bag as it's heated by the sun. The same principle is at work when you fog up a mirror with your breath.