Question: How Do Trick Birthday Candles Work?
Answer: Have you ever seen a trick candle? You blow it out and it 'magically' re-lights itself in a few seconds, usually accompanied by a few sparks. The difference between a normal candle and a trick candle is what happens just after you blow it out. When you blow out a normal candle, you will see a thin ribbon of smoke rise up from the wick. This is vaporized paraffin (candle wax). The wick ember you get when you blow out the candle is hot enough to vaporize the paraffin of the candle, but it isn't hot enough to re-ignite it. If you blow across the wick of a normal candle right after you blow it out, you might be able to get it to glow red-hot, but the candle won't burst into flame.
Trick candles have a material added to the wick that is capable of being ignited by the relatively low temperature of the hot wick ember. When a trick candle is blown out, the wick ember ignites this material, which burns hot enough to ignite the paraffin vapor of the candle. The flame you see in a candle is burning paraffin vapor.
What substance is added to the wick of a magic candle? It's usually fine flakes of the metal magnesium. It doesn't take too much heat to make magesium ignite (800° F or 430° C), but the magnesium itself burns white-hot and readily ignites the paraffin vapor. When a trick candle is blown out, the burning magnesium particles appear as tiny sparks in the wick. When the 'magic' works, one of these sparks ignites the paraffin vapor and the candle starts to burn normally again. The magnesium in the rest of the wick doesn't burn because the liquid paraffin isolates it from oxygen and keeps it cool.