In the airport I hit on this worth noticing toilet flush!
Though it looks as a normal toilet flush, it works this way (as the writing says): you push the button once and it starts flushing, you push it a second time and the water stops.
I find it really clever for many reasons:
- it uses a common interface in a different clever way - users already know how button toilet flushes work and if they do not care to read, the device works all the same;
- it puts the instructions in a place where they will surely be noticed: rarely there are writings on toilet flushes xD;
- it conveys the message (the device functioning) in a simple, cross-language, "visual" way;
- it strengthen the message of "water saving" through the use of the drop;
- it empowers the user - now he is in control of the amount of water to use, so he should use only what's needed.
Great great example of essential design.