Yea I would always just use body heat, like gtrain said, tight boxer briefs, would use one of the bigger visine bottles and would keep it in the junk area for a couple of hours before testing, flipping it over every once in a while, and never had a problem. A tip someone mentioned that I never had to use is if the thermometer strip on the cup doesn't register once you empty the contents into it, to piss on it from the outside, which he claimed worked.