We have probably seen this example of a USB stick that swells up when full, and deflates when empty.
Although this is a mockup that apparently has been going around for quite a while, I do think these sort of designs are interesting examples how interaction with technology can be made more intuitive.
This phone made by Samsung is real:
It’s screen displays a water level that corresponds to the level of ‘juice’ left in the battery.
NTT DoCoMo in Japan just started selling the N702is 3G cell phone to its subscribers. The phone has a built in motion sensor. When the screen saver is activated, the screen displays glass-like water. The “water” moves when you move the phone around. The water level also reflects your current battery status, the lower it reaches, the less power you have.