It seems counterintuitive, because anesthetics are supposed to knock out patients and make them numb to all the jolts, jabs, and twinges of surgery. But a new study shows that many anesthetic drugs actually stimulate parts of the nervous system that sense pain, and can lead to increased discomfort after surgery. Surgeons already knew that some knock-out drugs cause a burning sensation at the site of injection or in the lungs if the drug is inhaled, but the new research illuminates the mechanism of that response, and helps explain postoperative pain that was thought to be a result of the surgery exclusively.