Play your drum under the moon and your hair will swell! The more you play and the longer it's under the moon, the larger your hair will swell.
Using this method, your hair swells so much it looks like a giant bowling (ぼう) ball is sitting on top of your head. It's all round, and well... swollen is probably the only word that fits.
Currently there are no similar looking kanji which exist in the WaniKani learning system