You might initially think that the answer is 1/2, however, it isn't.
Before I tell you anything, all you know is I've flipped two coins, and the possible outcomes are:
HH, HT, TH, TT
I now tell you that there was at least one head, which eliminates TT, leaving three possibilities. Of these three possibilities, only 1 allows the second head: HH. So the probability is 1/3. QED.