A disappearing smile could help save the lives of thousands of babies across the developing world, its makers say.
The image completely changes when the temperature falls
The smile is seen on a disc stuck onto the skin which monitors temperature.
When babies are warm enough, mothers will see a green spot with a smile - but if they are too cold, the smile disappears and the spot turns black.
The team of medical and thermometer experts who devised the device say hypothermia is a major cause of death among newborn babies.
The LCD (liquid crystal display) ThermoSpot temperature indicator was developed by John Zeal, whose family firm has been making traditional mercury thermometers since 1888.
He was approached by Professor David Morley, a child health expert, who set up the charity Teaching Aids at Low Cost (TALC), which sends low-cost healthcare, teaching and training materials to the developing world.
With the help of other child health and LCD experts, Mr Zeal devised ThermoSpot - which is bright green when a baby has a healthy temperature of between 36.5ºC (97.7ºF) to 37.5ºC (99.5ºF).
But if the temperature drops below that, the colour fades. The ThermoSpot, which is 12mm in diameter, turns black when a baby's temperature hits 35.5ºC (95.9ºF).
At that temperature, if a baby is not warmed quickly, it is likely to die.
'Easy to use'
The LCD disc has an adhesive back, so it can be stuck securely onto the skin.
Studies, including one published in the Lancet, have shown that ThermoSpot match readings from rectal thermometers.
Its makers say the device could be produced for as little as 10p.
The disc has now been entered for this year's Medical Futures Innovations Awards.
Mr Zeal told the BBC News website: "If the background is green, and you can see a smiley face, all is well.
"But the moment the temperature goes down, the background fades, and the mother will know the baby needs to be warmed."
He said three separate studies were currently underway, and once those results were in, he hoped the World Health Organisation would give its backing to the device.
A team of researchers from the Embangweni Mission Hospital in Malawi, who tested the device, said in a Lancet editorial: "Hypothermia is a common and serious problem in the developing world.
"Very low nursing staff-to-patient ratios prohibit regular temperature monitoring with glass-mercury thermometers.
They added: "ThermoSpot is a simple, accurate device allowing continual thermal monitoring of low-birthweight infants in a resource-poor setting.
"Poorly educated mothers quickly understood how the ThermoSpot worked and responded appropriately when warned of hypothermia."
Dr Patricia Hamilton, a neonatologist at St George's Hospital in London, said: "Hypothermia in newborns can be a problem. In Nepal, for example, it's not the custom to wrap babies up after birth."
But she said it would be better to teach mothers to keep their babies warm.
"This device may have some use as a teaching aid to show mothers babies can be dangerously cold.
"But it is better to get the message across that babies should be wrapped up."