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If you say that something you hear goes in one ear and out the other, you mean you quickly forget it.
左耳朵进右耳朵出;当作耳边风
If I have to listen to something I don't understand, it just goes in one ear and out the other.
要是非得让我听我不懂的事情，那也只是左耳朵进右耳朵出。