One of the most well known facts of nature is that hot air rises. Together with the hot air, water vapor also rises. High up in the air, where it is very cold, this water vapor condensates or ‘form small droplets of water’ which we perceive as clouds and before we know it, it rains!
When air is hot, its atoms are farther apart than when it is cold, and so it takes up more room. That makes hot air lighter than cold air, because the cold air has more atoms in the same amount of room or ‘volume’. As the air is heated by the candle’s flame, it rises and causes the foil spiral to turn.