Conjuring Trick

I recently came across another blog (I'll point you to that blog at a later date), which has a number of puzzles on it. One, in particular caught my eye (probably because of a long time fascination with conjuring).

The trick works as follows.

A volunteer chooses five cards from a normal pack of cards. You take the five cards, look at them and return one of the cards to the volunteer. The other four cards are handed to another volunteer. You leave the room and a colleague enters. The second volunteer reads out the cards you gave them and your colleague names the card that the first volunteer is holding.

How is this done (post your ideas)?

If you want to know, click the "interesting" box at the bottom of this blog and, if there is enough interest, I'll point you to the original blog and the answer (unless somebody already knows, of course).