An ancient elven stone of magical property is missing and the PCs are trying to find the thieves and return it.
A dubious NPC is boarding a ship to leave the elven lands for the human lands across the sea. The PCs suspect, but are not 100% sure, that this guy is the one who's got the stone. He carries a sack at his side that radiates magic. The stone could be inside...
They're more than willing to pummel the thief to get the stone back, but are afraid to pummel the wrong person. Confirmation is needed.
The Paladin PC in the group strides bodly up to the NPC as he is boarding the ship and demands he empty his sack and show him the contents. The NPC is outraged and refuses. The Paladin is at a loss for words....he can't convince this guy to open his bag. After repeating his demand often and after the NPC continues to refuse, the Paladin lamely comes up with an alternative. The Paladin knows the stone is roughly the size of an apple. So he thinks that if he's got something in there shaped like an apple, he's gotta be the one....
So he says, "Can I feel your sack?"
Needless to say, we all busted up laughing (including the Paladin). Now, when any PC is at a loss for words when talking to an NPC, no matter what the context of the conversation, the players just say, "....well......can I feel your sack?"