Cooks have known for thousands of years that a low cooking temperature provides the greatest safety margin for making custards: that is, it gives us more time to recognize that the dish is properly done and remove it from the heat, before it toughens and tunnels. Custards are usually baked in a moderate oven with the protection of a water bath, which keeps the effective cooking temperature below the boiling point. The actual temperature depends on the pan material and whether and how the water bath is covered (see box). It’s a mistake to cover the whole water bath, since this forces the water to the boil and makes it more likely that the custards will be overcooked. The most gentle heating results when the individual molds are set covered on a rack in an open, thin metal pan of hot water.