4M Potato Clock
What, no batteries? Be a scientist as you discover how to power this digital clock using potatoes. Experiment to find out what other substances will power the clock - you'll be amazed! Caution: High Voltage Inspiration and Fun! Includes digital clock with wires, pots, connecting wire, transparent tapes, copper and zinc strips and detailed instructions.
Recommended for ages 8 and up but if you know a clever little tike, I'm sure they'd give it a good nudge, mate!