Your partner leads the 7 of Diamonds against 3NT. Before you play to the first trick, stop and review the bidding. Declarer opened 1 Club and rebid 2NT, promising 19-21 points (they don't have to hold what they promise, but it's a a good place to start.) Can you count his tricks? One thing you do know for sure is that he has five club tricks, whether he holds the KIng or whether your partner has it. If you defend passively, will the contract make? The answer is almost certainly yes. If your partner holds the King of Clubs, declarer holds the Queen of Diamonds, and you have no hope, since declarer can always make at least nine tricks by finessing against the King of Clubs. So, mentally place the King of Clubs in declarer's hand, and the Queen and 9 of Diamonds in your partner's. Now the contract fails, but only if you act decisively. You play third hand high, the King of Diamonds. Declarer wins with the Ace and plays a club to dummy's nine, your partner playing the five. When he leads the 3 of Spades from the board, you must fly with the Ace and return a diamond. If you don't, the spade winner is declarer's ninth trick.