When it comes to heating up synthetic urine, there are a few things you need to take into consideration. The first is that most commercial brands of fake pee will come with heating pads. These can be used to bring the temperature of the sample up to body temperature, which is around 98°F.
However, some brands do not include heating pads. In this case, you'll need to find another way to heat the sample. One option is to microwave the urine for 10-15 seconds. Be careful not to overheat it, as this can damage the chemical composition of the fake piss and make it less effective.
Another option is to use a hot water bottle or other similar device filled with hot water. Simply place the bottle under your clothing, next to where you'll be storing the urine sample, for a few minutes before you plan on using it. This will help keep the sample at body temperature until you're ready to use it.
Once you've heated up your synthetic urine sample, it's important that you don't reheat it again later on. This can again damage the chemicals in the sample and make it less realistic.