>>I have often wondered this, occasionally looked for the answer, and never
>>really needed to know enough to work it out from first principles.
>
> Strange, I've never wondered about it o_O

Hehe, the exact problem I was applying it to was that I needed to save all 
the possible images generated by some website, I knew there was a certain 
number of possibilities and that it was random that each one appeared, so I 
just wondered how many times I would need to try before I got all of them.

I didn't need to know exactly, eg if N=100, am I expecting 150 tries, 200 
tries, 500 tries?  But knowing the exact formula would be cool as it seemed 
an interesting problem without an obvious result.

Another situation I can think of is if you need to check every item in a 
physical population, but you can only select ones at random (eg tagging fish 
in a lake or something).

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