Hard boil an egg - app. 5 minutes. Allow it to cool. Dissolve as much alum as will dissolve in about a tablespoon of vinegar. Alum is found in the pharmacy or in the grocery store with pickling supplies. Use this solution to write on the eggshell using a toothpick. Once the egg is dry, it will look perfectly normal. When the shell is later removed, the writing will be visible on the egg's surface. Note: although alum has been used for years in pickling, it is no longer considered as safe as it once was (aluminum compounds may be linked to Alzheimer's Disease) so it is best not to make a habit of eating too many of these eggs. Also note that this trick doesn't always work. It seems to depend on the egg used. I am going to do some experiments with different brands of eggs (free range and otherwise), boiling times, etc. If and when I find the "secret" to making this work every time, I will add it here.
Above is from Bizarre Stuff You Can Make in Your Kitchen