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straining one's ears
子供はこっそり聞き耳を立てた。
The child secretly pricked up their ears.
猫が遠くの音に聞き耳を立てる。
The cat pricks up its ears at a distant sound.
隣の夫婦喧嘩に、彼は聞き耳を立てていた。
He was straining his ears to listen to the quarrel next door.