Newton's Second Law of Motion
Newton's Second Law of Motion tells us that
For every action there is an equal and opposite reaction.
Everything moves relative to everything else. If you push somebody, the push forces her backwards, but it also forces you backwards - that's the equal and opposite reaction. Sometimes the thing you're pushing against is so big that you don't notice the reaction (like if you are pushing off from the ground, the Earth is too big for you to see the reaction), but it is still there.
To find out more about movement, check out these books from Amazon.com or from your library:
