Participants are tested for sensitivity to warm and hot temperatures. A probe is applied to the skin (usually the forearm) and heated to temperatures ranging from room temperature to that of a hot cup of coffee. Participants rate the temperature using a scale ranging from "no pain" to the "worst pain imaginable" and rate the unpleasantness of the heat using a similar scale. The probe is applied up to 30 times, using random heat intensities. Participants are also asked compare the heat intensity to varying levels of sounds and to rate the magnitude of different sensations they have experienced in the past, such as the brightness of the sun and the loudness of a jet plane. A blood sample of about 2 ounces is also collected at this visit for DNA analysis to look for genes related to pain.