Have you ever flipped a coin as a way of deciding something with another person? The answer is probably yes. And you probably did so assuming you were getting a fair deal, because, as everybody knows, a coin is equally likely to show heads or tails after a single flip—unless it’s been shaved or weighted or has a week-old smear of coffee on its underbelly.
So when your friend places a coin on his thumb and says “call it in the air”, you realize that it doesn’t really matter whether you pick heads or tails. Every person has a preference, of course—heads or tails might feel “luckier” to you—but logically the chances are equal.
Or are they?