Here is an activity that your teacher can show you.

A candle is stood in a holder and then lit.

As the wick burns, it heats the solid wax and the wax turns into a liquid and dribbles down the candle.

As the wax dribbles down it is cooled in the air and it becomes solid again.

When the wick stops burning the pool of wax around it turns into a solid. When the wick is lit again, the wax melts and a pool of liquid reappears.