Heat is measured with a thermometer. A thermometer is a glass tube that ends in A bulb containing mercury. Numbers are written on the tube. The mercury in the bulb heats up when it touches something hot, like the inside of your mouth. As a result, the mercury expands and rises up the tube. If we do not have a fever and are resting, the mercury will stop at 37.0 ºC (98.6 ºF). When we are unwell and have a fever, the mercury rises further, and the doctor knows how high the fever is.