Mysterious Fact?

This sounds mysterious, but it is a fact.

The offspring of two sets of identical twins are legally cousins, but...genetically siblings. 

I did not know this and had to fact check it to be sure - it just seemed so unbelievable at first glance. However; when you think it through it makes absolute sense.

LESSON - Don't go with your initial assumptions (remember what we've said about -DON'T ASSUME).  Think things through.