What's especially good this experience - all necessary for him probably have at home: candle, pharmacy iodine (in alcohol solution, iodine tincture) and some worthless iron object - an old door hinge, key from an unknown castle or the castle, the keys of which are lost.
The metal surface on which a drawing, grind emery cloth to Shine. Light the candle and tilt it so that the wax drip onto the shiny surface. Slightly heat the object, then the wax will be spread in a thin layer. And when it cooled and cools the needle scratch with a groove, so they came to metal. Type dropper pharmacy iodine and drip on the scratches. After a few minutes the iodine solution turns pale, and then again put it on the scratches. After about an hour, remove the layer of wax: you will see on the metal clear traces, they exactly repeat the pattern on the wax.
If the experiment was successful, you can move on to more serious business, not just scratching the wax, and write on it the word or draw a picture, for example, to mark his pocket knife or wrench on the bike.
Look what happens when iodine comes into contact with the metal. Iron reacts with iodine, the result is a salt of iodide of iron. And this salt - powder, which is easily removed from the surface. And where there were scratches formed cavities in the metal. This process is called chemical etchingPM To him often use, but usually do not use iodine and other substances, more active.
Incidentally, iodine interacts not only with iron, and copper. So, it is possible to etch the various objects of copper and copper alloys, for example brass. Can try.