What is the ruling on a young man and woman smiling at each other?

Young men and women must lower their gaze. If their gaze falls unintentionally, they must immediately avert it. It is not permissible to intentionally look and smile, as this is a gateway to temptation and corruption. It is forbidden for a woman to uncover her face or smile at a non-mahram man, due to the evil consequences this leads to.