Now I find out (according to Andrew Gelman at Statistical Modeling, Causal Inference, and Social Science) that a coin can't be biased - the probability of a flipped coin coming up heads MUST be 50%:
The biased coin is the unicorn of probability theory—-everybody has heard of it, but it has never been spotted in the flesh. As with the unicorn, you probably have some idea of what the biased coin looks like—-perhaps it is slightly lumpy, with a highly nonuniform distribution of weight. In fact, the biased coin does not exist, at least as far as flipping goes.Dang! I guess I'll have to get a new analogy.