It took me a while to understand that example. Basically if you multiply two intervals (-3,3) * (-3,3), it's relevant whether it's actually the same quantity m

It took me a while to understand that example. Basically if you multiply two intervals (-3,3) * (-3,3), it's relevant whether it's actually the same quantity multiplied by itself (in which case, you want the answer 0-9, because you'll always be multiplying something like -1 * -1, because you're sampling once from the interval and then squaring it), or it's two different quantities, both of which are in the same interval, in which case the answer you want is (-9,9), because you could be multiplying something like -3 * 3, because you're sampling twice from the same interval and multiplying the two results together.