If your expensive ear thermometer isn't accurate enough to tell whether your baby has fever, try your engagement ring. Researchers have used diamonds just 100 nm in diameter to detect temperature changes as small as 0.044°C inside living cells and similar-sized gold particles to produce those changes. When atomic defects in the diamond lattice, called nitrogen-vacancy color centers, are illuminated by green light, they emit red light with an intensity that depends on temperature. Monitoring this light produces a superaccurate, atomic-scale thermometer. The scientists used silicon nanowires to inject the gold nanoparticle and diamond crystals (shown in this artist’s impression as gray), into a living cell. While the gold nanoparticle was heated with a laser beam, the diamond thermometers monitored the exact temperature changes in different parts of the cell. The researchers, who report their findings in Nature, also determined how much heat was needed to kill the cell. They hope that the findings may allow basic insights into cell biology or even cancer detection, as malignant tissue's faster metabolism raises its temperature.