Why do horror movies always have pianos and mailboxes?

Because the piano and mailbox are heavy.

This is a humorous joke, because there are several demons in the piano and several demons in the mailbox.

Origin: Judging from the opening song "Wide and Wide Tian Tong Avenue" in the 82nd edition of Journey to the West, several demons have just been captured and knocked down. Ghosts, how can there be so many! Bai: Monster, eat my old grandson! ) kill the soul and lose the soul. God is trembling, and so are ghosts.