Here's why:
* We can't see gravity: We can't directly observe gravity, but we can see its effects.
* It acts at a distance: Gravity pulls objects towards each other, even when they are not touching. Think of the Earth pulling on you, or the moon pulling on the Earth.
* It's a fundamental force: Gravity is one of the four fundamental forces of nature, alongside electromagnetism, the weak nuclear force, and the strong nuclear force.
While we can't see gravity, we can see its influence in the way objects fall, the tides rise and fall, and the planets orbit the Sun.
