Two ordinary black slates, such as used to be used in schools in lieu of paper or personal computers, are presented to the audience and demonstrated to be blank on all sides. A question is asked and the performer intimates that s/he'll make the answer appear on one of the slates. S/he takes a small piece of chalk, places it between the slates, binds them together with an elastic band, and holds the package up in sight of the audience; a scratching sound indicates that writing is taking place. The performer then separates the slates and demonstrates that the answer has appeared on one of them!
The mechanics of this trick are quite simple. The answer is already written on one of the slates; the writing is covered by a closely-fitting sheet of cardboard, of the same colour as the slate (slate A in diagram #1). Slate B (the unrigged one) can be offered to an audience member to examine; likewise slate A; but a substitution must be done in sight of the audience. The performer takes slate A in his/her left hand between his/her thumb and forefinger, and slate B between the fore- and middle finger of the right hand: slate B being under slate A. The slates being brought together as in shuffling cards, when the two slates are exactly in line (superposed with each other) the right thumb and forefinger grasp slate A while the fore- and middle finger of the left grasp slate B. When done smoothly and confidently, the audience can't detect the finger switch, and slate B is again presented (by the left hand) for examination.
When the slates are bound together, the performer must have arranged things so that s/he knows whether slate A or B is uppermost. The scratching noise is produced by a finger under or behind the slates, depending on how they are held. When the elastic band is removed and the chalk shaken loose, the performer can separate the cardboard from the surface of the slate and hold it tight against the other while the writing is displayed! Diagram 2 shows the situation if the slates are laid flat at the moment of revelation, but this is for illustration only.
The most difficult part of this illusion is getting the answer on to the slate. The modern magician/hypnotist Reveen, in his similar trick of producing an oil painting on a blank canvas, recites a list of famous paintings before (apparently) casually saying "let's try for so-and-so". So-and-so is already freshly painted on the canvas and is revealed by the same method as described here. Likewise, either the performer must request an audience member to pose a question and write down the answer, which is given to an assistant who then prepares the slate while some indirection is going on — a difficult matter! — or some simple question can be posed, such as "which card have I selected"? The performer, being experienced, can pick a card from a rigged deck, or throw a loaded dice, or — if s/he knows the local history and stories well can get the audience to propose a few and by forcing pick the one that has already been written on the slate.