[**Third law:**](http://en.wikipedia.org/wiki/Newton%27s_laws_of_motion)

> 
> When one body exerts a force on a second body, the second body
>  simultaneously exerts a force equal in magnitude and opposite in
>  direction to that of the first body.
> 
> 
> 

Newton's third law is unintuitive to me. If the earth exerts a gravitational force on me, how do I exert a force on Earth? 

Why is this true? It doesn't make any sense. How did Newton come to this conclusion?