Well, there's two equally likely possibilities, and the one with heads is one of those two equally likely possibilities, so there's a 1 half chance. Same thing if we were to ask what is the probability of getting tails. There are two equally likely possibilities, and one of those gives us tails, so 1 half. And this is one thing to realize. If you take the probabilities of heads plus the probabilities of tails, you get 1 half plus 1 half, which is 1. And this is generally 2. The sum of the probabilities of all of the possible events should be equal to 1, and that makes sense because you're adding up all of these fractions, and the numerator will then add up to all of the possible events.