Special note: Why is it OK to have 0 on top (in the numerator) and not on the bottom (in the denominator)?
Consider for a moment what division means. The reason that is because 2·5 = 10.
The fraction because 2·0 = 0.
The fraction can't equal anything. There is no number you can multiply by 0 and get 10 as your answer. The fraction is undefined.
What about ? It's undefined, too, but for a slightly different reason. If you multiply the 0 in the denominator by any number at all you get the 0 in the numerator. It seems that can equal any number. As a result we say is indeterminate, which is a special kind of undefined expression.