A negative is one half turn and is applied by multiplying whatever you have it next to by the half turn. If a value is negative, it's heading the opposite direction of positive. multiplying by another negative turns you half way around a second time, and points you back at positive. this is the "a negative times a negative is a positive" If you are wondering if negative is really rotation, ask Euler, it's his Identity. On the other hand, if you are adding two negative numbers, that's just moving in the negative direction both times.. which makes it more negative. This is way easier to understand if the student experiences it first. Turning around and Walking it out is my favorite tools for this.