Sarah was born in the caul: that is to say, she was born while still  in the amniotic sac. Such babies look surprisingly unperturbed: covered in a shining skein, bathed in ichor, their healing moistness displayed for all to see. Normally, the sac is punctured, the  fluid seeps away and the baby soon looks like any other. In other, more superstitious ages (so Dickens tells us), the caul was thought to bring good luck, and more specifically to provide a protection against drowning. Sailors would vie with each other to buy one, and, with their prize locked away in their ditty-box, they thought themselves assured of a dry death.

Her caul looked like a protecting veil, and indeed as she matured, Sarah saw its softness and elasticity as a kind of armour. It grew with her, expanded as she changed, stretched as she became taller. She could see it herself: it was her shimmering alter ego. It knew how to prevent pain. It was autonomous. Its viscosity was sometimes a problem, and she had to take care where she sat, as she left damp patches everywhere. That could be embarrassing. But on the whole, folk were astonishingly unobservant, and failed to notice that this person in their  midst was covered by a soft carapace which protected her. 

Then things began to change. The law of osmosis asserted itself. The caul started to leak a little. Tiny ruptures appeared on its surface, and bodily fluids - sweat, tears  - seeped out through the gaps. She looked like a living fountain. But worse was to come.  Through the little cracks, the effluvia of the outside world breached her caul. Blood, excrement, bile, bacteria seeped through it: it was fully porous. It was a two-way process.

And that was how Sarah became human at last. Joy had to be mixed with pain, purity had to be mixed with danger. Nothing was all of a piece, was it? And gradually, the caul began to slip down, to slide from her body. There it lay, transparent, regretful, like a piece of soft gauze billowing round her ankles. Someone else might want it. But it would not save them from drowning, probably.