Ask a volunteer to roll three dice with your back turned to them.  Then ask them to add together the three values thrown.  Now, ask them to pick up any one of the dice and add to the total, the number on the bottom face.  Finally ask them to roll the die they picked up, again, and add to the total the number thrown.  You now turn to face the dice, pick them up, and baffle your volunteer by announcing the total they arrived at.

Because the opposite sides of a dice add up to 7 all you have to do is add 7 to the total of the three values shown on the dice once you have turned to face them.