Here's why:

* Discovery: A discovery is the act of finding something that already exists. Gravity was not created by humans. It's a fundamental force of nature that has always been present in the universe.

* Invention: An invention is something new that is created by humans. Gravity is not something that was designed or built.

Isaac Newton is credited with discovering the law of universal gravitation, which describes how objects with mass attract each other. He didn't create gravity, but he explained how it works.

So, while we can use our understanding of gravity to create things like rockets and satellites, gravity itself is a natural phenomenon we can't invent or control.