And finally tonight, we have been hearing from you all day long about something incredible we showed you last night. The tiny babies doing a kind of backstroke across a pool. You wanted to know so much more and so, here's abc's sharyn alfonsi. Reporter: William and ellie are like most 9-month-olds. They don't walk or really talk yet. But in the water, they are anything but ordinary. Swimming on their backs 25 meters -- about 27 yards. This underwater camera revealing just how confident william is. Watch as he flips himself over. His parents, vic and charley, calm. I thought all babies could do it. It wasn't until we were here and all the swimming teachers were so excited about it that we realized just how clever they were. Reporter: Is this natural act or instinct? Or can it be learned? It's a definite combination. Reporter: At new york's imagine swimming, they teach 6-month-olds how to swim. We know some other schools in the country that are pioneering and doing even at 4 weeks. Reporter:4 weeks old! Seem early? Studies show babies who swim develop better spatial awareness, coordination and fine motor skills. Can she actually swim with those cheeks? I mean, is that possible? Still, taking the plunge isn't always easy on a parent. The first time is very scary. You're like, no! But they float. Reporter: Instructors say they're struck by the twins' parents. Mom and dad are very relaxed in the water. They don't panic. Reporter: It seems the advice is the same farnts and babies. Take a deep breath, let go and relax. William and ellie have that part perfected, too. Sharyn alfonsi, abc news, new york. And do it with supervision.