We all know that siblings can have different fathers – technically making them half-siblings – but what about twins? Yes, it can happen. In fact, one study estimates that as many as 1 in 400 sets of fraternal twins is "bipaternal."

How is it possible? Simple: Two eggs from the same mother get fertilized by two different fathers – within the same ovulation period.

Okay, maybe not so simple. But if your head is spinning, consider that this happens in certain animal species all the time – with more than two fathers! Our canine friends are a clear example.

"Female dogs ovulate multiple eggs," says Joann Boughman, executive vice president of the American Society of Human Genetics. "And when a dog is in heat, any male dog in the vicinity will attempt to mount her, so each egg could be fertilized by a different father, producing a litter of puppies that don't resemble each other in the slightest."

Humans aren't dogs, of course, and most of the time, women ovulate only one healthy egg per cycle. Fraternal twins occur when women ovulate two healthy eggs and both get fertilized. And every now and then, "superfecundation" happens: Two eggs are ovulated during the same cycle and fertilized at different points within the six-day fertility window, resulting in a bouncing pair of twins.

When we see twins, we may assume that both eggs were fertilized during a single act of intercourse. But it's quite possible for one egg to be fertilized during one act of intercourse, and the other during another. For example, consider a couple that has sex in the morning, followed by another round that evening.

It's only logical, then, that when a woman has sex with more than one man while she's fertile, "heteropaternal superfecundation" can occur. That is, each egg can be fertilized by a different father.

However, if you know twins who look nothing alike, don't assume that they have separate fathers. Like any siblings, twins who have the same father can look completely different from each other. They may even appear to be of different races.