Caffeine crystals. Coloured scanning electron micrograph (SEM) of anhydrous caffeine crystals (1,3,7-trimethylxanthine). They were produced by a process called sublimation. A liquid containing caffine, such as coffee, is frozen and heated to 238 degrees Celsius, causing the frozen liquid to vaporise without going through the liquid phase. The vapour is then condensed, which drives the water out and results in anhydrous crystals. Some of the crystals have symmetrically intergrown (upper centre, red and yellow). Caffine stimulates the central nervous system (CNS), increasing alertness and deferring fatigue. It occurs in coffee beans and tea leaves. Magnification: x400 at 10 centimetres high.