The Relative Box was an invention created by Professor Hubert J. Farnsworth. It's purpose was to determine if two people were related to each other after putting their fingers in holes on either side of the box. The light bulb would light up if there was a relation.
It was used by Hubert, in December 31, 2999, to confirm that Philip J. Fry I was indeed his many times over great uncle. It is actually shaped like a bone.