The Metronome is one of the most unusual attacks from the popular animé/cartoon series and video game, Pokémon, having a totally random effect each time it is used.
In the Pokémon Red, Blue and Yellow games, the Metronome performs one TM attack at random. In Pokémon Gold, Silver and Crystal, Metronome performs any attack at random, not just TMs.
Clefairy learns metronome naturally at Level 31 and Mew learns it at Level 30. Metronome can be taught to the following Pokémon using TM35 (found on Cinnibar Island): Clefairy, Clefable, Poliwhirl, Abra, Kadabra, Alakazam, Geodude, Graveller, Golem, Gengar, Drowzee, Hypno, Chansey, Mr.Mime, Jynx, Electabuzz, Magmar, Snorlax, Mewtwo, Mew and all fighting type Pokémon.
As seen in a number of episodes of the animé/cartoon series, "just about anything can happen" when the Metronome is performed. In this case, the usage of the Metronome is not specifically limited to a specific named attack - in one episode, Togepi manages a huge explosion of some kind by way of this attack, although which attack this actually was isn't entirely clear.
Pokémon seen in the series to use the Metronome include Togepi, Clefairy and Drowzee.