When you drop a ball to the ground, what makes it fall?
Gravity! Our gravity comes from our planet Earth always pulling us back toward the ground.
Gravity is a type of acceleration, and the longer we fall toward the Earth the faster we'll go.
So if you fall from an airplane, you'll start out falling slow,
then you'll go faster and faster until you pull your parachute and slow yourself down.
If you drop a ball from the sky, it will fall slow at first, then faster and faster.
(from: wikipedia - gravitation)
