Gravity doesn't *pull* the picture shelf down. It's actually the weight of the picture shelf and anything placed on it that causes it to be pulled down by gravity.

Here's a breakdown:

* Gravity: Gravity is a force that attracts everything with mass towards each other. The Earth has a lot of mass, so it exerts a strong gravitational pull on everything around it, including you, your picture shelf, and even the air you breathe.

* Weight: Weight is the force of gravity acting on an object's mass. The heavier an object is, the stronger the force of gravity pulling it towards the Earth.

* Picture Shelf: Your picture shelf has weight because it has mass. The weight of the shelf itself, combined with the weight of any pictures or decorations you put on it, creates a downward force on the shelf.

* Support: The shelf is usually supported by brackets or a wall mount. These supports resist the downward force of the weight, keeping the shelf from falling.

If the supports are not strong enough to resist the combined weight of the shelf and its contents, then the shelf will indeed fall down due to gravity.