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It's hard to escape childhood without racking up a few scars. Why do these leftover reminders of a painful cut or crash look different from the rest of our skin? And why do they stick around for so long after the incident that caused them? Sarthak Sinha explains how scars form.

4:33 video, For 10,000 years, humanity suffered from the scourge of smallpox. The virus killed almost a third of its victims within two weeks and left survivors horribly scarred. But Simona Zompi commends the brave souls -- a Buddhist nun, a boy, a cow, a dairymaid and physician Edward Jenner -- who first stopped the spread of this disastrous disease, to make us smallpox-free today.

For 10,000 years, humanity suffered from the scourge of smallpox. The virus killed almost a third of its victims within two weeks and left survivors horribly scarred. But Simona Zompi commends the brave souls - a Buddhist nun, a boy, a cow, a dairymaid and physician Edward Jenner - who first stopped the spread of this disastrous disease, to make us smallpox-free today.

The treatment used olfactory ensheathing cells (OECs) - specialist cells that form part of the sense of smell. OECs act as pathway cells that enable nerve fibres in the olfactory system to be continually renewed. In the first of two operations, surgeons removed one of the patient's olfactory bulbs and grew the cells in culture. Two weeks later they transplanted the OECs into the spinal cord, which had been cut through in the knife attack apart from a thin strip of scar tissue on the right. They had just a drop of material to work with - about 500,000 cells.

Sonnenburg, his wife Erica, and the graduate student Samuel Smits confirmed this idea in a recent experiment. The researchers started with mice that had been raised in sterile bubbles and then loaded with identical collections of gut microbes. They then fed these mice a high-fiber diet, before randomly switching half of them to low-fiber chow for seven weeks. Predictably, the fall in fiber caused upheavals in the rodents' guts. In the low-fiber group, the numbers of 60 percent of the local microbe species fell dramatically, and some remained low even after the mice returned to high-fiber meals. Those seven low-fiber weeks left lingering scars on the animals' microbiomes.

Starfish can regrow lost arms, and salamanders can sprout new limbs. So why can't we? Sci Show explains the science of regeneration, and explores the limitations the humans face -- and are trying to go beyond.