Newton's Third Law of Motion
Newton's Third 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 that the Earth is also moving away from you), but the reaction is still there.
Bibliography and further reading about Newton's Laws:
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