You hold two die. Throw them on the table. The bottom face of the dice you are not visible. Take the cubes and show these faces the viewer. Let him put you invisible glasses. Put the cubes aside and correctly name the desired sum of the two bottom faces.
For this you need to know that playing dice, the sum of opposite sides is always equal seven. If on the one hand 2 points, then, with the other 5. You can see the upper side of the cubes.
For example, from your side 4 and 1, i.e. a total of - 5. And the total sum of the two opposite sides on the two dice is equal to 14. So, to name the sum, known to the spectator, 14 subtract 5. Name it - 9. Because the faces of the cube, which was seen by the viewer, were 3 and 6 points.