Suppose I come to you and offer to play the following game. I toss a fair coin. If it comes down heads, I give you £5; but if it comes down tails, you give me £10.
This is not a favourable gamble for you: the expectation is that I will win £2.50 — in other words, if we play this game n times, I will win a total amount that, as n increases, approaches £2.50 x n.
But suppose you just think, what the heck, let’s do it, and as it happens the coin comes down heads. You win £5 and walk away.
Now: when you decided to play, did you make a good decision?