Emulsifying
Fat and water, which usually split, stay bound in a smooth sauce.
What to do
- Start with an egg yolk, mustard, or a spoon of sauce you already trust.
- Add the oil in a thin stream while you keep stirring.
- If it splits, beat it back in, spoon by spoon, to a fresh yolk.
A common slip. A whole bottle of oil at once, and the sauce breaks into fat and water.