Stratum Corneum
Your skin's outer wall is 20 layers of dead cells. That dead layer is what keeps you alive.
Grounded in the research on Stratum corneum (Wikipedia)
What it actually is
The stratum corneum is the outermost layer of your epidermis: roughly 10-30 micrometers thick, built from 15-20 stacked layers of corneocytes, which are flattened dead cells with no nucleus. It is the literal barrier between you and the outside world. Everyone obsessing over skincare is fighting over this paper-thin sheet of dead cells.
The brick-and-mortar model
Dermatologist Peter Elias coined the model: corneocytes are the bricks, an intercellular lipid matrix is the mortar. The mortar is ceramides, cholesterol, and free fatty acids arranged in stacked bilayers. That lipid mortar, not the cells, is what actually blocks water loss and pathogens. Strip the lipids (harsh surfactants, over-exfoliating) and the wall leaks.