Potara

Potara are earrings worn by the Supreme Kais and their attendants for generations. Despite being worn so casually, they are actually an incredible power-up item.
These earrings are worn by all Supreme Kais as part of their standard outfits, and come in various colors. When one pair of Potara earrings are worn by a single person they have no special properties. However, if two individuals each wear one earring on opposite ears, the true power of the artifacts is revealed. The earrings have the ability to fuse two people into a single entity, substantially enhancing the abilities of both individuals. The resulting fusion lasts forever, even if the fused being removes the earrings, so caution is needed when handling them. However, there is one instance where the Potara Fusion will be nullified and restore the fused being back to their original forms, which is when they are sucked into an environment that would not support it (as seen with Vegito being absorbed and touched by the air inside inside Buu).
Old Kai gives his pair to Goku as a trump card for defeating Super Buu. In addition, the Eastern Supreme Kai and his attendant Kibito merge together without knowing a thing about Potara fusion.
Potara Fusion is similar to the Fusion Dance technique, however unlike the Fusion Dance, the Potara Fusion is permanent. An exception to this is discovered when the fused character Vegito allows himself to be absorbed by Super Buu and is separated into Vegeta and Goku, the two individuals who had fused to form Vegito. Also, while with the Fusion Dance the post-merged clothing is the native dress of the people of Planet Metamor, when merging with the Potara not only are the two people's bodies mixed together, but their clothing is as well.