The base game is super simple. You go around taking turns, each player saying a London underground station - or pick a similar, familiar, enumerable set - and the first one who says Mornington Crescent wins the game. When someone does, you quickly announce the winner and immediately start a new round.
If you play it with children, see how long it takes them to realize that the fun lies in not saying the winning station. And how much fun you can have by anticipating what the others are going to say.