We have probably seen this example of a USB stick that swells up when full, and deflates when empty.
(source: www.smallsurfaces.com)
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:
?
(source: www.mobilecowboys.nl)
It’s screen displays a water level that corresponds to the level of ‘juice’ left in the battery.
From www.slashphone.com:
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.