Here's why:

* Newton's Third Law: This law states that for every action, there is an equal and opposite reaction.

* Your Action: You exert a downward force on the floor due to your weight (mass x gravity).

* The Floor's Reaction: The floor exerts an upward force on you, which is equal in magnitude to your weight. This upward force is what keeps you from falling through the floor.

To calculate the force:

1. Find your mass: You can weigh yourself on a scale.

2. Multiply your mass by the acceleration due to gravity: On Earth, gravity is approximately 9.8 m/s².

Example:

If you weigh 70 kg, the force the floor exerts on you is:

70 kg * 9.8 m/s² = 686 Newtons (N)

Important Note: This is a simplified explanation. The actual force might be slightly different depending on factors like the angle of the floor, if you are moving, or if you are applying additional force to the floor.