A little bit of science is brought into everyone's life on that fateful day they try to blow out birthday candles while making a wish, only to see the candles do the impossible and reignite.
It's a day we learn two things. 1.) The laws of nature are mysterious; and 2.) Parents are cruel. Find out the trick in trick candles.
First, let's talk about how regular candles work. Although candles only burn around the wick, it's not the wick that's on fire — it's the paraffin wax. The wick is just a delivery system. It absorbs the melted wax and brings it upwards. Once the liquid wax is drawn upwards by the wick, it is vaporized by the heat of the flame at the top of the wick. It is that vapor above (and sometimes around) the wick that ignites and makes the teardrop-shaped flame that we see on candles.
The wick itself does get hot, but while the candle is going, it doesn't burn. The evaporation of the paraffin wax cools it down, the same way the evaporating sweat on skin cools the body down, preventing the wick from burning out quickly. When the candle is blown out, that hot wick still vaporizes the wax. The white stream of smoke that comes up from an extinguished candle is that vapor. If someone were to hold a match to that vapor, it would ignite, the flame would travel downwards, and the candle would reignite.
Trick candles manage that reignition without another candle being near. A flaming candle needs three things: fuel, oxygen, and a certain amount of heat to get started. Once it absorbs that heat, it gives off still more heat and keeps going on its own. Different materials burst into flame at different heat levels. The materials in a regular candle (the wick and the paraffin) need a lot of heat to start burning. Magnesium needs less heat to burst into flame. It can ignite using only the heat from a still-smouldering candle wick.
Trick candles include magnesium in their wicks. When the candle is going strong, the magnesium is coated in liquid paraffin, like the rest of the wick. The liquid paraffin keeps oxygen away from the magnesium, making it impossible for the magnesium to combust. The evaporation of the wax makes it extra hard for the magnesium to burn. But when the flame is put out, the paraffin coating the wick vaporizes away, but there is no flame to melt more of the candle and create more liquid wax. The wick is exposed to oxygen, and because it's still cooling, it's hot enough for the magnesium to spark. When the magnesium catches fire, the heat it gives off is enough to reignite the paraffin vapor and make the candle flame again.
And that's how science stole your birthday wish.