If you are a bit fuzzy on Newton's and Einstein's theories of gravity, this short video from NOVA explains it clearly.
Gravity Lesson Plans!
Science Buddies has a great lesson plan for understanding gravity, similar to the experiment that Galileo did from the Leaning Tower of Pisa. Watch Galileo's classic experiment re-enacted in this BBC video.
Bright Hub Education offers a lesson plan titled, "An Experiment on the Center of Gravity." Kids can compete against adults and win at these activities. Fun activity for your class.
Hot air balloons are fascinating to kids. This lesson plan from the Smithsonian's Air and Space Museum lets students build their own hot air balloon as they model lighter than air flight. Middle school teachers may want to check out this Helium Balloon Race lesson plan.