How to create a fair coin from an arbitrarily biased coin:

1. Toss the coin and remember the answer.

2. Toss the coin again, if it is different from your previous toss then your result from #1 is fair. Otherwise, go back to step 1.

If p is the probability of getting heads, there are four possible outcomes with their associated probabilities:

    TT -> (1 - p)^2   (rejected)
    HT -> p * (1 - p)
    TH -> (1 - p) * p
    TT -> p^2         (rejected)

Needless to say, p * (1 - p) and (1 - p) * p have an equal probability, so if we don't reject our two tosses, we have a fair outcome.