At the end of the year, each of 12 friends posts a greeting card to every one of the others. How many cards are posted altogether?

Each friend posts a card to the other 11, so the count is $12\times 11=132$. The traps: 66 halves that figure, which is right for handshakes — one handshake serves both people — but wrong here, because the card A posts to B and the card B posts to A are two different cards; 144 is $12\times 12$, letting every friend post a card to himself as well; 121 is $11\times 11$, which drops a whole friend from the count.