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msp1672 says ...

Butt of the joke????  I don't think so.  I think that all of a sudden you found your maternal gene with this child and mom was congratulating you. . . .albeit left handedly.

I have found it interesting that when children are small and someone hands you their child. . . . .I have held it at a distance hoping that it doesn't leave it's mark (i.e. some sort of bodily fluid) on me.  When my daughter was small. . . . .I couldn't hold her close enough and that "mark" thing didn't seem to bother me.  I think you have experienced this in somewhat of a small way.  While the child is not yours, you have accepted the child as "yours" and are willing to do what is needed to look out for the child as if it where your own.

Not being a psychology major, I may be way off base but for now it's the best I can come up with.

I like this diagnosis ^^^^^^