Ask a volunteer to roll five dice.  Then you magically tell them the total sum of the five dice faces that are facedown on the rolling area and hidden from view. Once you have done this, ask your volunteer to turn each die over in turn and total the bottom faces as they go. The total will be the same as the total you announced previously.

You can do this by simply adding up the five dice values that are uppermost and subtracting this number from 35, because opposite sides of a die always add up to 7 and 5 x 7 = 35.

You can do this trick with any number of dice by simply subtracting the total of the  uppermost faces of the dice from 7 times the number of dice you are using.