One of Galileo's most important discoveries was about gravity.
This is the force that makes things fall down.
Before Galileo, there were no scientific studies of gravity.
People generally believed that heavy things fell faster than light things.
Galileo showed that this was not true.
In his most famous experiment, he used a heavy ball and a light ball.
He let them both go at the same time so they rolled down a board.
The heavy ball didn't roll faster than the light ball.
They both rolled at the same speed.
From this experiment, Galileo understood an important law of physics: the law of falling bodies.