Our bodies are giant sacs of blood and guts. But unless we are impaled, we don’t spring a leak and empty out everywhere. It may seem impossible that sweat can leave but other fluids cannot. Science, however, has the answer to how we keep it all on the inside.

Read: How to Reattach a Severed Limb

DNews host Jules Suzdaltsev explains that it is the skin’s top layer, the epidermis, that forms the tight seal we need. The epidermis’ own top layer — “It’s layers within layers; it’s skinception,” Suzdaltsev jokes — is the one that sheds millions of cells every single day, and beneath that, the stratum granulosum, is the seal of freshness. For the seal to work, the geometry of the cells there is key: Their structure is something called a “tetrakaidecahedron,” a 14-sided mouthful of a shape that has six rectangular faces and eight hexagonal faces.

Lord Kelvin, the 19th-century mathematician, physicist and engineer who is partly famous for having a unit of temperature named for him, is the one who first imagined that complex shape. The skin cells are flattened versions of it that fit together without leaving any gaps, according to recent research. “The cells in this layer are sealed together in a zipper-like fashion.”

Suzdaltsev compares the way old cells are replaced with new ones in this cell design to a conveyor belt on assembly line — old ones move down the line to the next station as new ones are continuously created in the lower levels of the skin and come up on the assembly line after them. When our skin sheds throughout the day, “old skin cells at the top disengage while keeping a redundant seal beneath them, then pushing the new skin cells forward with another seal behind that.”

Source: Yokouchi M, Atsugi T, van Logtestijn M, et al. Epidermal cell turnover across tight junctions based on Kelvin's tetrakaidecahedron cell shape. eLife. 2016.

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