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It is a warm summer's day. The birds are chirping in the air. And you are walking down the street, whistling away. You find a pebble by the side and throw it up. It falls back onto the ground. Why did this happen?

What goes up, must come down.

The ground is the natural resting place for a pebble.

Your gentle throwing hasn't given the pebble enough kinetic energy to escape the Earth's gravity