Ask someone to balance three dice, squarely, on top of each other to form a tower.  You then tell them that you can magically guess the sum of the five faces hidden from view.  The face on the bottom of the tower and the four faces where the dice touch.

Because opposite sides of a die always add up to 7 you know the faces of the bottom two dice add up to 14.  The number on the top die, facing down, is 7 minus the value of the upper face.  Or to put it another way, it would be three 7s (21)  minus the top face.  Look at the dice tower and feign concentration as if you can see through the dice to the sides hidden from view.

To astound your audience again you can repeat the trick with an extra die.  Simply add another 7 to the total for every extra die used.