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now that you mention it; come to think of it; that reminds me; on that subject; speaking of which
そういえば彼女の誕生日が近いことを思い出した。
Come to think of it, I remembered her birthday is coming.
そういえば昨日の会議はどうなったの?
Come to think of it, what happened at yesterday's meeting?
そういえばあの店は今日休みだったはずだ。
Come to think of it, that store should be closed today.