Gravity is a very, very weak force.  Back in 1798, Cavendish used a torsion balance to try to estimate the value of G, the gravitational constant.   What’s cool is that you can reproduce this experiment yourself.   Check out Bending Spacetime in the Basement, which includes some nifty videos of the torsion balance in action.

[tags]Science,Gravitation,Science Fair[/tags]