Gelatin

Gelatin comes from bone and skin, cooked down slowly until it dries into sheets or powder. Drop it in cold water and it just sits there, drinking, going soft and springy in your hand. Warm it and it disappears into the liquid. Let that liquid cool again, and it grips — a spoon dragged through it meets resistance, a jelly holds its shape on a plate, a panna cotta trembles but doesn't run. That wobble is the whole point.

Look closely before you buy. Good sheets are smooth and even, the same colour from edge to edge, easy to separate without sticking together in a lump. Powder should look dry and fine, pale to lightly golden, never damp or clumped — and it should smell of nothing, not of the fridge, not of anything old.

Some people lump gelatin in with flour or cornstarch. Don't. Feel it set — firm, clean, easy to unmould, with a bounce a starch never gives you. The mistake is treating it like a starch, boiling it hard and long on the stove. Do that and it loses its grip; you'll pour your jelly into the mould and watch it stay soft, wondering where all that hold went. Get it right, and it melts the moment it touches your tongue — that's the reward for treating it gently.