Here's why:

* Gravity: Your body is pulled towards the center of the Earth by gravity. This force is your weight.

* Contact Force: When you lie on the bed, your weight presses down on the mattress. This is a contact force, as it requires direct contact between your body and the bed.

So, while you might feel like you're simply relaxing, your body is still exerting a downward force on the bed due to gravity.