How two negative numbers multiply to give a positive number?
Imagine a rocket ship which can be either pointing up (positive) or down (negative). The rocket ship has forward thrusters (positive) and reverses thrusters (negative). Now if the spaceship is pointing down (negative) AND fires its reverse thrusters (negative) it goes UP (positive). So negative and negative make positive. Other permutations are possible. For example, if the spaceship is facing down and fires its forward thrusters it goes DOWN. Therefore, a negative and a positive make a negative.
