The tool is simple enough to pass hand to hand: a forearm-length bar of fired clay, inlaid with strips that shift color with moisture and heat. Press it to soil, a wall, or a stored heap of food, hold it for a slow count, and the dent it leaves and the color of the strips tell you whether the thing under your hand is holding steady or quietly changing. It spread fast because it answers a very old worry with one deliberate pause.

But ask Vraezan, of Bamako, what worry drove him to build it, and you reach a wall. He remembers what the tool is and can hold that in his mind. The question of what uncertainty he was trying to resolve does not come back to him when he reaches for it.

Vraezan refused to invent an answer that would have sounded right. What he found instead was a seam in himself. He seems to care about precision in reading conditions, and the tool itself shows that much. But he cannot say what he was afraid of, or what he was protecting against.