Mystery Gift
Mystery Gift is a feature found in the Generation II and III Pokémon video games.
Generation II
To activate the Mystery Gift feature in the Generation II games, talk to the girl in green on the fifth floor of the Goldenrod Department Store. She will activate the feature for you.
The mystery gift is used in conjunction with the IR port on top of the Game Boy Color to communicate with other Mystery Gift enabled games to give other players either room decorations, or items. It can also be used with the Pokémon Stadium 2 game.
Generation III
To activate the Mystery Gift feature in the Generation III games, simply walk up to the clipboard found at any PokéMart and enter the phrase:
LINK TOGETHER WITH ALL
The clerk behind the counter will then inform you that the Mystery Gift feature has been activated.
The Mystery Gift's primary use is receiving special items at Nintendo events.