Wax does not melt back on the candle. The wick is lit and as it is burning, the heat melts the wax just below the flame. The melted wax is what is burning and it will burn slowly and as it does it leaves and goes into the air as gas. Then, the wick is able to burn further down melting new wax and the process continues. That's why there is generally a pool of liquid wax at the base of the flame. You can also try this. Use a piece of string and put it in a small piece of foil so that it is half sticking out above and below the foil. Then make the foil like a little boat and float it in some vegetable oil with half the string soaking in the oil The string will burn at first and then it will begin to burn the oil at the surface. It will demonstrate the principle for your project. Get help with an adult please to do this.