You're choosing one term from each thing you're multiplying by.
It's similar to the idea of the binomial theorum.
For example...
(x+1)(x+1) or (x+1)^2
If you were to fully expand it.
x*x + 1*x + x*1 + 1*1, in each pair, you're choosing 1 term from each "x+1".
There is only 1 way to get an x^2, choosing x for the first pair, and then choosing it again in the second pair.
There are 2 ways to get an x, choosing x first pair then a 1 second pair, or 1 first pair and x second pair.
I hope that made sense. :))
=^._.^=