I've come up with a riddle that I use on a daily basis to solve a simple problem.
In a kitchen, you have a water tap that can run cold and hot water (or a mixture). The more fancy ones have a handle that can rotate in 360 degrees where horizontal movement determines the temperature, and the vertical movement determines the tap being on or off.
It is possible to determine how warm the water is without touching the water itself or using any device that measures the temperature that will work on most, if not all water taps (I have not found a tap where this did not work)
So, how can you determine if the water is cold, warm or hot without touching the water?