I have just found a very cool gadget: the On/Off Mug.
When the mug is full full of hot liquid the cup turns white and is emblazoned with the word ON in black letters, and as it cools it turns black and clearly screams OFF at you in white letters.
This is fun and can be a good gift idea.
When it is half full it looks like this.
On Off Mug Features
The price of On/Off Mug is £15.99 (aprox. $25.55) and it is available at Iwantoneofthose.com