Babies: Natural swimmers?

Yo!
Have you ever seen a baby float in water? 
They say their natural swimmers, but why?

If babies are natural swimmers then why do some people don't know how to swim do they need to be retaught or have they forgotten about it? Babies are incredible really from a soft cuddly piece of responsibility to somewhat dependable people (opinions may vary). so what makes them good swimmers? Babies now how to walk from birth,it's only because their legs can't support them enough to carry their body. But swimming, really? How does a person forget how to swim. It's like the basic of basics next to reading.

While were on the topic of babies. Babies are a pain in the butt to raise. I have two nephews and they are troublesome as heck. though the rewards are good enough you can say."Hey, I helped raise that kid whose gonna grow up to be a respectable adult(results may vary). So about babies being natural swimmers. Still I don't know how. May because they're light-boned like birds? I don't know.

Well that's it for now. Have a good day.