Here's why:

* Gravity is a universal force: Every object with mass exerts a gravitational force on every other object with mass.

* The strength of gravity: The strength of this force depends on the masses of the objects and the distance between them. Larger masses exert a stronger pull.

* The Earth's gravity is just one example: While the Earth's gravity is strong enough to keep us on the ground and hold the moon in orbit, other objects in the universe exert gravitational forces as well. For example, the sun's gravity holds the Earth in orbit, and the moon's gravity causes tides on Earth.

Let me know if you'd like more examples!