It is a couple with brown hair who have a child. Only problem

It is a couple with brown hair who have a child. Only problem, it turns out to be red. Intrigued, they go to see a doctor to ask for an explanation before considering other hypotheses.
The doctor asks them:
- Do you make love every how long? Everyday ?
- Ho no, no ...
- Every week ?
- No no...
- Every month then?
- No, no, not even.
- So every two months?
- No no...
- Every six months then?
- Yes, that's about it.
- So, look no, it's rust!
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