Volunteers: Volunteers will participate in two 90-minute sessions in which they will receive and rate four heat stimuli per minute applied to the skin for a maximum of 36 minutes. The heat stimuli range from 37° (Degree)C to 51° (Degree)C (99° (Degree) F to 124° (Degree) F) and last 2 to 3 seconds. A drug commonly used in dental treatments may be administered during the second session. This will be either a maximum of 0.15 mg fentanyl, a short-acting narcotic pain killer, or a maximum of 5 mg saline, an inactive substance (placebo).