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Flies don't fly into a closed mouth.
Literal
Closed mouth's inside [to-に] flies [topic-は] fly-into not.
A proverb: 閉じた口の中に蝿はとび込まない — 'flies don't fly into a closed mouth.' The lesson: keep your mouth shut and trouble won't find you. Japanese equivalent of 'silence is golden' or 'loose lips sink ships.' 蝿 is the kanji for 'fly' (the insect).