This is one of the first riddles I was able to remember consistently and I got it from my Dad. A man looks at a portrait and says, "Brothers and sisters, I have none. But that man's father is my father's son." What is the relationship of the man in the portrait to the speaker?
The official solution is that the man in the portrait is the son of the speaker. However, an anonymous commenter pointed out that the man in the portrait could be the son of a deceased brother of the speaker, making him the speaker's nephew. This depends on interpreting "Brothers and sisters I have none" as leaving open the possibility that the speaker once did have a brother.