Proving the binomial coeficient is always a natural number?

How would one go about proving that:

is always a natural number? That is, assuming and that and that ; what I mean by that is that and are both positive integers. Its a question in my textbook that I bought early to get ahead on fall classes, so I don't have any teachers to ask about this question; not till fall atleast. I'm not asking for a straight out proof, but preferably some direction as to where I would go, which direction I should go to prove this? A litttle guidence is all I need, please don't simply give me the full answer, if you don't mind. Thanks in advance