The success of this demonstration depends upon creating an illusion that you are working with an ordinary candle. The room must be darkened. You may use an apple in place of a potato. If more than three minutes will elapse between the time the apple is cut and the presentation of the experiment, the outside of the apple will need to be treated with lemon juice to prevent darkening. It is debatable as to whether or not this is a good practice.
Raw apples are easier to ingest than raw potatoes.
The match and burning material can cause fires and burn injuries. Keep a fire extinguisher nearby. Use caution with the razor. The materials may be disposed of with ordinary solid waste. Wrap the used razor blades carefully before discarding so that janitorial personnel will not be injured.