Your skin is a brick wall. A split is what happens when the mortar dries out.

The outer layer of your skin, the stratum corneum, is fifteen to twenty layers of dead, flattened cells stacked like bricks and held together with a mortar of lipids. It's only as thick as a sheet of paper, but it's the whole reason you can hold a crimp. Healthy, it carries ten to twenty percent water and bends like leather. Drop that water content and it stops bending. The first hard load on a dry pad doesn't stretch it. It cracks it, usually along the fold where the skin flexes most.

Chalk is the part climbers forget. Magnesium carbonate is a desiccant, which is exactly why it works on a hold and exactly why it keeps pulling water out of your pads for hours after you stop. Add a dry heater, a long drive, or a desert, and the mortar is gone by morning.

The repair kit climbers carry today came from the hardware store and the hospital. Cyanoacrylate, the stuff in super glue, was discovered at Kodak in 1942 and sprayed on wounds in Vietnam before anyone thought to put it on a flapper. Cotton tape with a decent adhesive came out of sports medicine. The balms came last, once people figured out that skin rebuilds overnight on roughly a four-week cycle and that the fastest way to speed it up is to give it water, lipids, and darkness.