This topic is in the 'Action and Reaction Forces' section. You've probably heard people say that every action has an equal and opposite reaction, and that's a quick and dirty way of stating Newton's third law. The key idea is that forces always come in pairs. Take a simple example, like when you lean against a wall. Are you pushing the wall or is the wall pushing you? The answer is that you can't tell, and for that reason one force can't exist without the other, and both forces are equally strong. This idea has far reaching consequences and you will learn about them in the tutorials in this section.