Kuchisake-onna appears as a woman with her mouth hidden behind a mask, asking "Am I pretty?" If you answer yes, she removes the mask to reveal a mouth slit ear to ear, and either answer is said to be hard to escape.
It is grounded in a real event: a rumor that began around Gifu Prefecture in 1978–79 and grew into a nationwide panic. Spread mouth to mouth until it became a real-world fear, it is a prime case of lore in which the rumor itself is contagious.