...I'm talking about cats, of course!
Have you ever wondered how a litter of kittens can look so wildly different from one another? Me too! So I asked a vet, and the answer is superfecundation.
When a lady cat is in heat, she will release multiple eggs, each potentially becoming a new kitten, but only one at a time. This process can last anywhere from a few days to a couple of weeks, and because of this, if she gets around during that time (as cats do), it's possible for every kitten in a litter to have different fathers. Or for some to be noticeably older than the others, even though they are all born at the same time.
Dogs can do this too.
It can even happen in human twins, though it is extremely rare. That's not stopping my neighbors from trying though!