Thursday Puzzler

Here's a puzzle for you. If I get some responses, maybe we'll make a full-fledged meme out of it.

A police detective is tracking known criminals in an attempt to locate their secret hideout. He follows two men to a warehouse. The first man approaches a door and presses a button next to the intercom. "Twleve," says a voice on the intercom. "Six," the suspect replies. The door swings open and he enters the building.

The second suspect approaches the door and presses the intercom button. "Six," comes the voice out of the speaker. "Three," replies the suspect. The door is unlocked, and he enters the warehouse.

"Oh, this is too easy," the detective says to himself. "It's almost as though these guys want to get caught."

With one hand ready on his gun, the detective leaves his hiding place and presses the intercom button. "Ten," says the speaker. "Five," the detective says in his best criminal voice.

Seconds later, the detective is surrounded by criminals who stand circling him, guns aimed at his head.

What should the detective have answered?