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

Concept: In a blood-relations puzzle, unpack the statement from the innermost relation outward and draw the family tree using standard symbols — square = male, circle = female, a double line = married couple, a single horizontal line = siblings, and a single vertical line = one generation gap. Placing every intermediate person named in the statement onto this tree lets the final relation between the two people be read off directly.

Application

1. "My uncle" is a relative of the girl one generation above her — her parent's brother.

2. "The father of my uncle" is one generation above the uncle — the girl's grandfather; call him X.

3. "The daughter of the father of my uncle" is a daughter of X — a sister of the girl's uncle, shown in the tree linked to Girl's Uncle by a sibling line.

4. "He is the son of" that daughter of X, so the boy is placed one generation below her, per the vertical generation-gap line joining her to Boy.

5. The tree also joins Boy and Girl with a sibling line, placing the two of them in the same generation.

Cross-check: Retracing the tree top-down — X's daughter (one generation below X), her son the Boy (one generation below her), and Girl joined to Boy by the sibling line — every link matches the statement, and the two generation-gap steps land Boy and Girl in the same immediate family.

So the boy is the girl's brother.