けす
to turn off
Water wears away at a thing until there is none of it left. The same key けす as 消える turning in the lock, except that this time your thumb does it on the way out: けす.
消える きえる is the light going out by itself. 消す is you turning it off.
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Emaki is not affiliated with, or endorsed by, Kaishi 1.5k or its authors. The word, reading and meaning come from the Kaishi 1.5k deck, used with their permission. The mnemonic, the note and the kanji breakdown are original work, published under CC BY-SA 4.0.