Consider it your portable public service announcement messaging system. Your mood and general workplace "readiness" is depicted on the side of the cup. As the coffee level goes down, and your caffeine intake therefore goes up, the people around you will know whether or not it is safe to approach you.
When you first get going, the level will read "go away", the next stop along the way is "Not Yet", then finally, you end with "How Can I Help You?"
For those who require more than a single cup of coffee or tea in the morning, you can give yourself a refill before it is completely empty to stay within the "safety zone".