The story received over six million hits on Youtube, and there are lots of reasons we may find it irresistible: the cuteness of the animals involved; the "surprise" of a cat taking care of what would normally be potential prey – a baby squirrel; and the exotic-ness of raising a wild animal.
It's not unusual for an animal to take care of another's young – cooperative or social breeding is common in the animal kingdom, from ants to birds to primates . In these situations, usually relatives help care for the family babies, assisting mom by providing various levels of care.
Cross-fostering, or the care for another species, is not as common in the wild. One key exception is the case of brood parasites, birds such as the cowbird or cuckoo, who lay eggs in the nest of a different species, leaving the foster mom no choice but to care for their babies . While not "naturally occurring", many experiments have also shown that animals of one species will care for the offspring of another.
So why (if you are a bird or cat or other animal...hmmm, even human) would you put time and energy into caring for another animal's offspring? Isn't the point to promote your own genes? It could be that the risk of not caring for a hungry face that presents itself to you is greater than the cost of doing some extra nursing or care, just in case that baby animal has some of your genetic material. Hormones may play a key role in this as well, as oxytocin produced in mother cats after kittens are born help make caregiving a priority  – and this caregiving may extend to baby squirrels, if they are presented at the right time (while the mom is nursing her own babies). And we humans, well, we are very susceptible to cuteness, which could in part explain why we take pets into our homes (but that's a topic for another time!).
And of course, when humans are involved, we are supplying most cross-fostering animals with supplemental care (be they mom cats caring for other kittens, puppies or squirrels, or animals living in labs) – and the costs of feeding another mouth are pretty small.
The big question – did the baby squirrel learn to purr?
So while we know mama cat provided this baby squirrel with nourishment, did she teach him to purr as the headlines claimed?
While true purring is attributed mainly to feline species, they are not the only animal that makes a purring sound. Purring sounds have been reported in foxes, racoons, badgers, and bears ! The main difference is that a true purr involves a several key characteristics which include occurrence during both inhaling and exhaling. Respiration during purring is higher than normal, and the frequency of purring is lower than normal vocalizing. The analysis of the "purring sounds" of other species indicates that they are different enough in sound structures that they should not be called "purring."