Ian Benton sends in this submission of a solar-powered device with an interesting backup mechanism in the event of a power failure:
" Iwas once given a combined tape measure and solar powered calculator like this one. I thought it would be handy for working out the area of a room. I measured the length, and entered it into the calculator memory. I then measured the width of the room. In doing so, my hand covered up the solar panel that powered the calculator, which caused it to lose the number in its memory.
Now I know why it includes a notepad and pencil! I suppose I could use the calculator to convert inches to millimetres, but the tape is already marked in both imperial and metric."