In the Fifties, a desk lamp in an interrogation room would be turned so that a perp would have the light in his eyes and stern questions would come out of the dark as sweat poured down his face. Sometimes, there might be a nice, cool glass of water just out of the perp's reach to "aid" his concentration. This was Perp Sweating.
There has been some escalation since the Fifties. Now the perp has to [...] [tvtropes.org]