★ riddle: pluck the strings for Wei Zi on his birthday.

Mystery: Playing the strings for Wei Zi (playing a brand name)

Answer: fireworks

"Birthday" can be understood as "ceremony". Of course, people should give "gifts" for their birthdays. "Wei Zi fiddled with the strings" is actually giving a "gift".

"Wei Zi" should have meant a person, and there is a "Wei Zi" in Princess Zhu Huan. Here, "Wei Zi" means "flowers", which is also called full house.

"plucking the strings" can be understood as "playing" here. Since it is "plucking the strings", it is "playing the piano".

Therefore, "playing the strings for Wei Zi on his birthday" can be understood as a combination of "ceremony", "flower" and "bomb", that is, the answer is "fireworks bomb". However, the "bomb" here is not a piano, but a bomb.