The "third law" says that if one thing puts a force on another thing, the second thing also puts a force on the first thing. The second force is equal in size to the first force. The forces act in opposite directions. For example, if you jump forward off a boat, the boat moves backward. For you to jump forward, the boat had to push you forward. Newton's third law says that for the boat to push you forward, you had to push the boat backward. Often, people say, "For every action there is an equal and opposite reaction."