It's not the pull of gravity that directly makes surface material move. It's the difference in the pull of gravity, which we call gravity gradient. Here's how it works:

* Gravity acts on everything: Gravity pulls on every single piece of matter, big or small.

* Uneven pull: If the pull of gravity is stronger on one side of an object than the other, that object will be pulled towards the side with the stronger pull. This difference in pull is called a gravity gradient.

* Movement: This uneven pull, the gravity gradient, is what causes surface material to move.

Here are some examples:

* Hills and mountains: The top of a mountain is further from the Earth's center than the bottom, so the gravity gradient pulls material downwards, causing erosion.

* Tides: The moon's gravity pulls on the Earth's oceans, creating a bulge on the side closest to the moon and another on the opposite side. This difference in pull causes the tides to rise and fall.

* Plate tectonics: Gravity plays a role in the movement of tectonic plates. Denser, older plates sink into the Earth's mantle, while lighter, newer plates rise to the surface. This process creates a gravity gradient that pulls the plates towards the Earth's center.

Key point: It's not just the pull of gravity itself, but the *variation* in that pull, the gravity gradient, that makes things move.