Keeping the cooking liquid

How to tell it is going right

• It was actually kept: the liquid went into a container rather than down the drain, and the volume saved is enough to be worth using.
• It is clean: strained of solids and grit, and not clouded with the debris of whatever broke up in it.
• It tastes of what it cooked and tastes concentrated rather than washed out, and it is not so salty or so bitter that the only honest use is the bin.
• It was cooled quickly and stored cold, not left standing warm on the stove to be dealt with later.
• Used, it does the job the dish gives it (carrying flavour into a sauce, loosening the dish, or setting when chilled) rather than merely adding water back.

Where it goes wrong The pot was left to cool slowly on the stove and only strained the next day, so it soured overnight: what should have been the best ingredient in the dish goes in the bin, and the recipe gets water instead.