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Newton's third law states that every action has an equal and oposite reaction. You may not think this, but when you're standing ont he floor, you are excerting a force against the floor: your weight, caused by gravity, is pulling you down. The floor is reacting by pushing back: we call this force the reaction force. 

Newton's first law states that if all the forces cancel each other out, nothing will go anywhere. But if your weight was stronger than the reactive force that the floor can supply, you'd fall right through!