This is aHand:

You watch the bidding as your opponents reach game in Hearts. 

You have an awkward lead. After that you have to think where four tricks might be coming from. 

East opens 1♥ after three Passes with 15 points and a semi-balanced hand. When West raises to 3♥ he happily accepts the invitation and bids game.

You decide to make the attacking lead of the ♠K from ♠K Q × and continue with the ♠Q after partner signals strong encouragement with the ♠10. You may as well try your luck with a third round of Spades but declarer ruffs.

Declarer draws trumps in two rounds and cashes his Diamonds, finishing in the closed hand. Now  comes the critical point of the hand as declarer plays the ♣4 towards dummy's ♣K J

This is a key moment. You play the ♣3 as smoothly and calmly as you can. Playing the ♣A would resolve declarer's guess in the suit. Even hesitating would tell declarer you have something to think about which may be clue enough in a situation which is a straight 50-50 guess. 

This is an oft recurring situation at this game, the famous "King-Jack guess".  Defenders should do their thinking well in advance and not be caught on the hop at the critical moment. They should Count-Plan-Execute as well as declarer. Here, you don't know North has the ♣Q but you lose nothing by hoping.

With nothing to go on except his horoscope declarer plays the ♣J from dummy and you are pleased to see partner win with the ♣Q. Another Club follows to your ♣A and declarer is one down. Yes, some days declarer will guess correctly and play dummy's ♣K and make his contract. If he does that: good luck to him – he will have earned his success.