Short answers, pulled from the story.
Finger wrinkling in water is caused by blood vessels constricting in response to signals from the sympathetic nervous system, not by the skin absorbing and swelling with water as was previously assumed. One hypothesis, called the rain tread hypothesis, suggests the wrinkles may improve grip on wet surfaces, but a 2014 study could not replicate earlier findings that supported this benefit.
Each of the four fingers contains three phalanx bones: the proximal, middle, and distal phalanx. The thumb has only two phalanges, lacking a middle phalanx. In total, the human hand contains fourteen digital bones.
Children's fingertips that have been torn off can regrow in less than eight weeks, provided the injury occurs above the nail. Regrowth is driven by regenerative properties of the distal phalanx in youth and by stem cells in the nail that create new tissue. No regrowth occurs if the injury site is below the nail.
Polydactyly is a congenital condition in which a person is born with more than the standard number of fingers or toes. It affects between 4 and 12 people in every 10,000. Some extra fingers are fully functional; one documented individual with seven fingers stated they gave him advantages in playing the piano.
Fingers contain no muscles capable of producing movement, aside from tiny arrector pili. All finger movement is controlled by muscles located in the palm and the forearm, which transmit force through long tendons that run across the wrist and hand.
Fingertips have the highest concentration of touch receptors and thermoreceptors of any area of human skin, second only to the genitals. A 2013 study found they can detect nano-scale wrinkles on an apparently smooth surface, a level of sensitivity not previously recorded.