This is a video of the Electron Plumber (better than plumbing turds) blowing out a tiny LED like a candle. How does he do it? *flourishes wizard cape* MAGIKA. "It's not magic." But it could be. "No it couldn't." Sleight of hand? "No." Make sense of all this then:
[electron_plumber] uses an interesting property of diodes (which are the "D" in LED) to use the LED itself as a temperature sensor. A diode's voltage drop depends on two things: the current that is being driven through the diode, and the temperature. If the current is held constant, then the forward voltage drop changes reliably in response to temperature. Turning the LED on warms it up and blowing on it cools it off, causing measurable changes in the voltage drop across the device. The change isn't much -- only a handful of millivolts -- but the effect is consistent and can be measured.
It's a clever demo with a two important details to make it work. The first is the LED itself; [electron_plumber] uses a tiny 0402 LED that is mounted on two wires in order to maximize the temperature change caused by blowing on it. The second is the method for detecting changes of only a few millivolts more reliably. By oversampling the Arduino's ADC, an effectively higher resolution is obtained without adding any hardware or altering the voltage reference. Instead of reading the ADC once, the code reads the ADC 256 times and sums the readings. By working with the larger number, cumulative changes that would not register reliably on a single read can be captured and acted upon.
So the LED itself is acting as a temperature gauge -- fascinating. Obviously, the next time it's one of your children's birthdays and you forget to buy candles you know what to do. "Use LEDs?" What? No -- tell them it isn't their birthday. Kids don't understand dates, you can postpone it a couple days, or weeks even. I don't even think I had a birthday from four to seven.
Keep going for the video demo, but it's basically just what you see above a couple times in a row.
Thanks to Eric P, who informed me blowing out candles when it isn't your birthday is bad luck. Is that true?!