ひとりでに
ひとりでに · hitorideni · JLPT N2 · adverb
naturally
ひとり (alone) + で (by means of) + に (adverb) = done alone, by itself, automatically
ひとり means one person or alone. The で-に ending turns it into an adverb, so the action happens with no one doing it. Use it for doors that open on their own or things that happen naturally.