The performer displays a clear clock dial with a rotating clock hand. The hand is rotated freely as the performer demonstrates that the clear sheet on which the dial is printed is absolutely solid.
The hand is then set to any hour called by the spectator and a pencil or large needle with ribbon or silk is pushed through a star trap in the clock hand and through the solid sheet.. It is then removed and the clock hand rotated to show that the sheet is completely solid again.
This is totally mechanical in working and can be performed no matter what time the audience call out.