From Glitch City Laboratories
- A Pokémon that evolves by evolution stone, such as Clefairy, Staryu, or Pikachu, as the first Pokemon in your party.
- One of the following Pokemon, each one of which corresponds to an evolution stone, in your party.
- Exeggutor - Moon Stone
- HEX 20 Missingno - Fire Stone
- Psyduck - Leaf Stone
- Growlithe - Thunderstone
- Onix - Water Stone
- The player has to get into a trainer battle.
- Have the first Pokémon in the player's party, the one he or she is trying to evolve, gain a level.
- Switch out the Pokémon that the player is trying to evolve and put the Pokémon that corresponds to the necessary evolution stone.
- Have that Pokémon finish the trainer battle.
This glitch can also be done when encountering a wild Pokémon. Have the Pokémon your trying to evolve start the battle and then change Pokémon and place the Pokémon that corresponds to the required evolution stone in battle. Have the Pokémon end the battle. Both Pokémon will gain experience and for the glitch to work, the Pokémon the player is trying to evolve will need to gain a level from the experience it gains.
One should be precautious not to let this glitch happen to the starter Pikachu on Pokémon Yellow because it will force it to evolve regardless of the fact that this is normally impossible as it refuses to take the Thunderstone. However, the player could always purposely evolve the Pikachu to make either gameplay easier or fill in Raichu's Pokédex entry but a Raichu with the player's OT will not follow the trainer.
Why It Works
The reason this glitch occurs is because the two operations, detecting a used a item and detecting which Pokemon is in battle, happen to share the same variable. The variable is 01XX91CF for Red and Blue, and 01XX90CF for Yellow. Use a Potion, and this variable is filled briefly by "14". Enter a battle, and send out a Pidgey, and the variable will become 24.