Heart Gold and Soul Silver: RageCandyBar
The RageCandyBar, also spelled RAGECANDYBAR or Rage Candy Bar, is a key item that you can buy at the west end of Mahagony Town before you go to Lake of Rage. In the original Gold and Silver the item would heal 20 HP. There is no obvious use to the item in Heart Gold and Soul Silver, but if you take the RageCandyBar to a man in the Underground Path that links Cerulean to Vermilion he will give you TM 64 which contains Explosion.
Trivia: The reason the RageCandyBar is a key item is so it can't be traded to Diamond, Pearl, and Platinum because there is no data for RageCandyBar in those games.
You may also want to know how to find the other TMs and HMs