Extraction theory, properly

Roasted coffee is only about 30 percent soluble — the rest is cellulose that no kettle will ever dissolve. Of that soluble third, the sweet spot is taking roughly 18 to 22 percent of the grounds' total mass into the cup. That figure — extraction yield — is the single number underneath every brewing decision you have ever made. Grind, temperature, time, agitation, ratio: all five are just levers on how much dissolves, and how evenly.

Below the window, coffee tastes underdeveloped: sour, salty-sharp, thin, a finish that vanishes. Above it, overdrawn: bitter, drying, hollow. Inside it — sweetness, clarity, and a finish that lingers without scratching.