Make sure the strip is free from any leftover packaging material. A liquid crystal thermometer, temperature strip or plastic strip thermometer is a type of thermometer that contains heat-sensitive (thermochromic) liquid crystals in a plastic strip that change colour to indicate different temperatures. Unpackage the thermometer strip. Read the thermometer while it is on the forehead. Look at the boxes and check the colour. There should be some plastic on the back of the strip that you will have to remove. ★ How to use the Forehead Thermometer: Hold the thermometer firmly at both ends and press flat against the middle of a dry forehead. There should be some plastic on the back of the strip that you will have to remove. Be sure the surface of the strip is free from any debris and is dry. Liquid crystals possess the mechanical properties of a liquid, but have the optical properties of a single crystal. (Use a mirror to read the thermometer on yourself). Hold in place for at least 15 seconds. Introducing by Tempagenix™ The first all-paper disposable forehead thermometer on the retail market Temp-N-Toss is a non-invasive, easy-to-use disposable forehead thermometer that conveniently fits in your pocket, purse or glove box and provides a temperature in just seconds, so it’s easier than ever to check for a fever! Step 3.