On chins

The human body is a surprisingly malleable machine. Although components like bones and tendons seem fairly rigid, they are in a constant state of flux. The body maintains a single pattern, but its constituent elements turn over continuously.

(SCIENTIFIC INTERLUDE: The only obvious exception to this of which I can think off-hand, not being a biologist (THANK GOD!!!), is neuronal DNA. Although neurogenesis (the growth of new neurons) has been observed recently in the dentate gyrus (the first segment of the hippocampus, which itself is essential to the formation and recall of new memories), it was long thought that new neurons are not formed in the adult brain at all. There is an exception to every rule in the brain, but the lack of significant neuronal growth is a well-established property. I can imagine just about every other component of the cell being degraded and replaced, but my impression is that, with the exception of a base pair here and there which are subject to error-correction, DNA remains untouched. Any actual biologists should feel free to correct me on this; I don't feel like looking it up.)

Even more notable is the adaptability of biological development programs. The human genome is not long enough to specify the position of every cell in the body. Instead, it contains (relatively) simple rules for building things up in a step-by-step manner. These rules adjust to the environment in which they are operating. If you encase a child's feet in tight bandages throughout their formative years, they will grow into an adult with tiny feet (also dysfunctional, prone to infection, paralysis, and muscular atrophy, according to Wikipedia, Prophet of Al Gore and Source of All Knowledge). If you cut your shins, separate them by a few millimeters, and fix them in place (an Ilizarov apparatus is recommended), the bone will grow to fill in the gap (at a rate of about a millimeter per day!) and you can add inches to your height.

Similarly, if you construct a triangular wooden frame and force your head into it every night during adolescence, you can develop the ability to impale people who irritate you using nothing but a swift downward motion of the head. It keeps the riff-raff in check.