Here's how it works:

1. You Bend Your Knees: You start by bending your knees, storing potential energy in your leg muscles.

2. You Push Off: You then forcefully straighten your legs, pushing down on the ground.

3. Newton's Third Law: According to Newton's Third Law of Motion, for every action, there is an equal and opposite reaction. So, when you push down on the ground, the ground pushes back on you with an equal and opposite force. This is what propels you upwards.

4. Gravity Takes Over: Once your feet leave the ground, gravity takes over and pulls you back down.

So, it's not that you are jumping "up" but rather you are pushing "down" and the ground is pushing you "up" in response.