Humans kissing. Thermographic footage of a kiss between a man and a woman. Scientists have discovered that blood flow is increased in the capillaries of the lips during a passionate kiss. A thermogram shows variation in temperature on the surface of an object, measured by the long-wave infrared radiation it emits. The temperature scale is colour-coded from black (cold) through blue, green and yellow to red and white (hottest). This thermogram footage also shows that the edge of the man's ears and the woman's hair are colder than the exposed skin.