evolutions
Clefairy is the first evolution, evolving from its baby form Cleffa with a high Friendship score, and evolving into Clefable with the use of a Moon Stone (just like the anime portrays).
I find this to be perfectly in line with what we see of Clefairy in the TV series--it is a Pokémon which prioritizes tight social bonds with its fellow Clefairy, and it venerates the Moon Stone as an object of worship. No wonder that Friendship values and the Moon Stone both play such an important role in Clefairy's development!
