My top ten most common answers:
10. Well actually, red hair is a recessive gene and since both of us have redheads in our families, it's not that unusual that Elijah is one.
9. My sister is a redhead.
8. He's a genetic throwback.
7. I don't know, but it sure attracts the girls.
6. I dye it.
5. He's actually a Weasley.
4. I don't know, I should ask the hospital about that one.
3. Does he have red hair?
2. His real father.
1. The postman.