Thank you for your help.
What is the better approach to answer this question?Using the concept of air or energy.

1)When heated, warm air rises and the movement of air causes the spiral to twirl.
2)Or, should we elaborate that heat energy from the candle is converted to the kinetic energy of air which causes the spiral to turn.

Sorry it took me a little longer to answer this question. Inherently some of this is obvious but I wanted to ensure that I was able to find support for my thoughts.

I believe the answer should be approached using the second solution. Why?

First, there is the obvious, this is a science question and thus we should approach the answer with science, in this case energy.

Next, the energy conversion does indeed start with the candle (flame) - the heat energy. The flame increases the temperature of the molecules in the air causing the molecules to vibrate more rapidly and move over a greater distance (kinetic energy). This results in the average space between the molecules increasing, and therefore the density of the air will decrease.

Now, contrary to the average thought, hot air does not rise. Technically, it is the cooler and denser surrounding air that moves downward by the force of gravity.

The confusing part is that we cannot look at a given molecule in isolation. It is the collective mass of cooler air that is pulled down by the force of gravity in comparison to the collective mass of warmer air which is lighter (less density) This gives the appearance of hot air rising.

It is this collective movement that allows birds to glide effortlessly up into the air. Or in this case, cause the spiral to spin.

Appreciate the thorough explanation by Chris... add some 'new' perspective to density of air. I feel it makes so much sense if we are able to share in such light with our children hence my comment:

1. It's so hard to answer a Primary School question in limited context yet we need to give the reference to the school syllabus and the maturity of the child.

2. The above lead to the challenge for us parents (untrained teachers) - how do we know what is the right 'dosage' of answer to provide? If it's purely for general knowledge it's easier but for scoring in a Science paper in Singapore - so much tougher!

3. With reference to Tianzhu's question - I noticed it is only a 1-mark question so I am likely to keep the answer simple PLUS, we need to also know which level is this question posted at and at that point in time, what topics have the school covered? Pr 4? Pr 5 or Pr 6??

Yes Daisy that is correct.

In fact, I coincidentally met with my daughters P4 science teacher yesterday and she commented that when she teachers the younger age she simply states that hot air rises. In the higher levels, she would answer the question using energy.