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Walls have ears, and sliding doors have eyes.
Literal
Wall [on-に] ears exist, sliding-door [on-に] eyes exist.
壁に耳あり障子に目あり is a well-known proverb meaning 'someone is always watching/listening.' The parallel structure (壁に耳 / 障子に目) is elegant and easy to remember. あり is the classical form of ある. This teaches both a cultural proverb and classical grammar in one line.