Divide two 24-inch by 24-inch poster boards into 4-inch squares. On one board, cut out each square, and leave the other as a grid. Using an ink pad, ink the two thumbprints of 36 people and place each set of prints in a cutout square. Place one side of Velcro on the grid and the other on the cutout. Velcro the cutouts to the grid.
With this display board it is possible to display all fingerprints, as well as take two off the grid and compare them individually. Encourage children to look for similarities in fingerprints, like an arch, a right-loop or a whorl. Also ask them if they can find two that are exactly the same. This is, of course, impossible, and will teach them that while they think they are exactly the same, there is always one part of a fingerprint that is different. Have an ink pad and paper available at the science fair booth so that participants can compare their own prints to those on the board.
Another board could display the history of fingerprinting, as well as how fingerprinting is used in the modern world. Attach a time line to the bottom of the fingerprinting board and end with references to how fingerprinting is used today. You can also use references to popular crime shows like "CSI" and "Law and Order" and compare them to real-world police work.
Offer a side experiment to show how police lift fingerprints from objects. You will need a Ziploc sandwich bag, a glass, and a tube of super glue.
Place the glass in the bag using the pads of your fingers. Put the bag flat on the counter and add a few drops of super glue to the bag, being careful not to touch the glass. Seal the bag.
In about 12 hours white fingerprints will appear on the glass, cause by the crystallized vapors of the super glue. This glass be brought to the fair to show the "after" effect, while the beginning of the experiment can be done in front of a live audience.