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shell (e.g. electron shell)
「殻」 has other readings with different meanings.
卵の殻を上手に割って料理した。
I broke the eggshell and cooked.
殻に閉じこもって出てこない。
He's shut himself in a shell and won't come out.
カニの固い殻を破るのはなかなか大変だ。
It's hard to break a hard shell.