Introducing a boy, a girl said, "He is the son of the daughter of the father…

Blood-relation puzzles are solved by resolving a compound kinship phrase from the innermost relation outward, replacing each kinship term with the actual family member it denotes. Whenever a step passes through a child of a grandparent or a sibling's child, the statement does not fix which sibling or which side of the family is meant, so more than one relationship can be equally valid for the same wording.

1. The innermost phrase is the father of my uncle. Whether the uncle is the father's brother or the mother's brother, his father is the same person as the girl's grandfather, so the father of my uncle denotes the girl's grandfather.

2. Next comes the daughter of that grandfather. A grandfather can have more than one daughter, and the statement does not say which one — she could be the girl's own mother, or the girl's aunt (her father's sister or her mother's sister).

3. Finally, the son of that daughter. If the daughter is the girl's mother, her son is the girl's brother. If the daughter is the girl's aunt, her son is the girl's cousin — specifically, the son of her father's or mother's sister.

4. Since both assignments of daughter are equally consistent with the statement, both resulting relationships are valid at once — the boy is either the girl's brother or her cousin (through that aunt).

Check it with concrete names: suppose the grandfather's two children are Meera and Neha. If the daughter in the statement is Meera (the girl's mother), her son is the girl's brother; if it is Neha (the girl's aunt), her son is the girl's cousin through Neha. Both assignments satisfy every clause of the original statement equally, confirming the wording never forces a single relationship.

So the boy is either the girl's brother or her cousin.