These LEDs are connected to a thermistor that’s running just a
little bit hotter than the ambient room temperature. So, by
blowing on the thermistor, the birthday boy or girl is cooling
it down, thus increasing the resistence. The microcontroller
senses this and turns off a few of the LEDs as a result. Make
one of these guys and you’ll never again have to worry about
melted wax on your cake. For detailed instructions, head on over
to Instructables.