Growth rate is a fundamental aspect of life, a metric that can separate biology’s winners and losers. Even a small advantage can lead to complete dominance in short order, given the exponential scaling patterns of biological growth. Calculating growth rate is pretty straightforward when you’re looking at plants or animals, where it’s possible to measure an organism’s mass with relative ease. But what about microbes? How can minuscule changes in a cell’s mass be measured when the whole organism weighs just a picogram (10-12 grams)?