The soup had cooked down too far while I was not looking. It had gone from a soup to something with opinions, thick at the bottom, the spoon standing up in it almost on its own when I let go.
You fix that with water, but not by measuring the water. You fix it by the spoon.
A soup tells you its thickness through the handle. You stir and you feel the drag, the resistance the pot pushes back up your wrist. Too thick and the spoon plows, it labors, it drags a trench that fills in slow behind it and the bottom starts to catch and scorch. You want the spoon to move easy but not free. A little weight to it. The liquid should close behind the spoon at about the speed it opened.
I added water from the kettle, a slosh, and stirred. Still heavy. The spoon still dug. Another slosh. The drag eased. The soup let the spoon through and folded shut behind it, unhurried, the way it should.
Too much water and I would have thinned the taste out to warm and nothing, all that cooking rinsed down to a broth you salt and salt and never fix. So I went slow. A little water. A stir. Read the handle. Again.
It came right somewhere I could not have marked with a cup. The spoon just started moving the way a soup moves, and I stopped.
I turned the heat down under it. A film had already set on the surface where it sat, and it wrinkled and slid when I dragged the spoon through it one more time.