Gelatine
Gelatine is an animal protein derived from collagen. Heated in water it dissolves; cooled, it forms an elastic, thermally reversible gel.
Its strength is measured in bloom: the higher the number, the firmer the gel at the same weight. Common grades are 160 (bronze), 180 (silver) and 200 (gold). Substituting one for another requires recalculating the weight, or the texture shifts.
Sheet and powdered gelatine work identically; only the preparation differs. Sheets are soaked in plenty of cold water and squeezed out; powder is bloomed in a measured quantity of water, usually five times its own weight.
The key limit is temperature. Above 60–70 °C gelatine begins losing its setting power, so it is added to a warm mixture rather than a hot one.
Fresh pineapple, kiwi, papaya and mango contain proteolytic enzymes that destroy gelatine: cook them first, or the gel will never set.