A bug-eyed creature on a small watch-sized screen jumps for joy when you earn points for being active and grows sick if you become too sedentary.
"When you lose points it looks really miserable," said Natasha Lee, a learning researcher who has been road-testing the gadget. "I've started walking in the evenings to keep up its health."
Once trials of the prototype are completed next year, the laboratory hopes the Fizzee will be bought by families and schools.
Its scoring system is based on advice from the Chief Medical Officer which states that children should be moderately active for at least an hour a day and exercise vigorously for at least three hours a week. A heart monitor strapped to the chest and a motion sensor contained in the device register heart rate and movement.
"A pedometer won't reward you for walking up a steep hill but a Fizzee will because it monitors exertion," said Ms Lee.
Each gadget will be callibrated to the user's height, weight, age and heart rate, and unhealthy children will see quicker results.
Ms Lee said that although the device could be useful for maths and ICT lessons, it might be hard to keep pupils in their seats if their Fizzees grew poorly.
"When a girl jumps up in class because her Fizzee is losing points, that is going to pose serious questions about how the school day is organised in terms of fitness," she said.
Primary weigh-in, page 24