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.