The coin flip, the ultimate 50-50 choice, is actually a little biased. According to a Stanford study, even a fair coin is about 51 per cent likely to land on the same face it started on. And if you spin instead of flipping, even a slightly weighted coin is much more likely to land on its heavier side. (A spun penny lands tails-up 80 per cent of the time.) But there's a cool maths trick for getting a fair result from an unfair coin.