So, today's engineering problem goes like this.


Someone in our family is dying their hair. The chemical (probably a Chromate-based hair dye) must be mixed with 180 ml of water at between 40 and 50 °C - if the temperature is too low, mixing is very difficult, too high and the compound breaks down, and perhaps you might not get the hair colour you need.


Simples ... except we have no thermometer.


How was this quick and easy to resolve?