You make a mug of tea or coffee. You get distracted. When you reach for the mug it's dangerously hot or disgustingly cold.
A simple stick-like device with a simple colour-coded tip. If the drink is too hot, the tip shows red. If the drink is too cold it shows blue. The Goldilocks Zone, not too hot or cold, shows amber.
The deluxe version would have a beeping function to audibly warn you that your drink was approaching 'too cold'.