The 77X42 (pronounced "Seven Seven X Four Two"), also referred to as the Super Sour Ball, is an extremely sour lemon ball.
Professor Frink exhibited the 77X42 at the Springfield Candy Convention. It was so sour, he said, that it could only be safely contained in a magnetic field. Frink then pointed at the 77X42 suspended in midair. Frink's warning, however, didn't deter Homer, who reached into the field, grabbed the sour lemon ball, and ate it without any hesitation.
Moments later, Frink noticed that the 77X42 was missing and asked what happened to it. Homer, with his face grossly distorted due to the sourball's tartness, mumbled, "I don't know."