LED Lights as messages

I have recent begun working at a consently busy Starbucks. There are many tasks to be done at a single given time. There are a coffee brewing tubs that have a small red LED light on the bottom. When this light flashes, it means that the coffee has become too cold, and, thus, is not fit to be served. I just wondered how a small system can detect the temperture of the coffee and light up the small light. Then, it got me thinking about how we use light everyday as signals or messages. Other examples are freeway signs, traffic signals, and even just looking at my computer screen right now! It's so exicting and, also, silly that most people in the US have just become so accustom to using light and electricity every day, when it is something so miraculous and so essential to our lives. It makes me appreciate light that much more!

(The Starbucks coffee machine looks similar to this but with a small red light on the bottom! I couldn't take a pic of it while at work. ahaha :P)