MODEL RELEASED. Measuring body temperature. Strip thermometer being used to measure the temperature of the forehead of a 9-month-old baby boy. There are liquid crystals in the strip that change shape and reflect light with changes in temperature. The different areas of the strip contain crystals that are designed to respond at different temperatures. The strip thermometer may show that the baby boy has a fever (a raised body temperature). A slight fever is mostly harmless, and helps the body to fight infection. However, a severe fever in a young baby requires medical attention.