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**SolCon** Still don't get it though. In your example, we keep flipping a coin until we get a head, after which we would stop flipping the coin. But only four flips are allowed. X is the number of tails. So, for the first flip, we can either get H or T. Both have equal probability. For the first flip, getting H would be the same as getting X=0 ('0' or 'no' Tail). But if we get T on the first flip, would that not mean X=1 ('1' Tail)? And seeing as their probabilities are the same (0.5), why would we multiply it by 0.5 a second time? That would mean a Tail and another Tail.