Preparation
Gather a feather, shallow dish, paper, two lemons, candle, lighter, knife, starch powder and two porcelain dishes. The lemons, feather and shallow dish are for invisible ink and the other items are necessary for fingerprint dust. Use caution when working with fire, especially if a child is helping you with this project.
Invisible Ink
Clean and wipe out the shallow dish, making sure it is completely dry. The "invisible ink" is only lemon juice, so it is important to keep impurities out. Cut each lemon in half and squeeze all the liquid into the dish. Remove any seeds that are in the dish to give your child the illusion that it is actually invisible ink. If you have the patience, you also may want to remove visible pulp because it may alter the final product.
Give the feather to your child and allow him to dip it into the lemon juice and write on a beige or cream-colored piece of paper. Although lemon juice appears clear, it will show a very small amount of color on bright white paper. The feather should have a moderate point on the end so that it mimics a pen point and is easy to write characters with. When the characters are dry, hold the paper over a flame or run over it with an iron. The lemon juice turns brown when it is heated and the characters are readable.
Fingerprint Dust
Put a small amount of starch powder on one of the porcelain dishes and hold a candle flame against the second dish. The flame deposits soot on the dish as it burns. Use a knife to scrape off the soot and add it to the starch powder until you have equal portions of both. Mix the two powders together until they are uniformly combined.
Apply the dust to a surface you believe has a fingerprint. You can apply it with a small paintbrush. The dust will absorb the oily deposits left by a finger and you can dust away the excess. You will see the shape of a fingerprint made of the dust.
If you want to keep the print, put a piece of clear packaging tape on the print and press down. Remove the tape slowly and stick it to a white piece of paper, which provides a bright background to illuminate the print.