Bend the end of the wire into a triangle, and tape to the desk. Bend the long straight end of the wire upwards. This is the stand for the spinner.
Cut the square piece of card stock into a circle (optionally using a compass to draw a circle on the card first). It does not have to be exactly circular, but make as large as possible.
Cut from the edge of the circle partways toward the centre, to make blades (telling them half way is fine if students might cut too far). Six or eight blades seem to work best.
Use a straight edge (ruler or book) to fold half of each blade partly upwards (see photo). Or fold by hand but make sure the crease is tight.
Make a small hole in the centre of the circle with the scissors (or pre-punch a central hole in each piece of card for students) then push the tube/cap through.
With the blades pointing down (may need to reverse the direction of the tube/cap), place the pinwheel on the end of the wire. Make sure the wire is not touching the edge of the tube/cap except at the very tip (keep friction to a minimum).
Use two small pieces of modelling clay to secure the candles on the desk under the pinwheel, making sure there is some space between the pin wheel and the candle (so the paper does not catch fire). One candle can work too if everything else is optimal, but starting with two gives the best chance of pinwheels working.
An adult lights the candle. Wait for the pinwheel to start turning. Do not leave unattended.
If a pinwheel does not turn, here are some troubleshooting ideas:
Make sure the wire tip is only touching the very end of the tube/cap. Any rubbing of the wire along the length of the tube/cap will create too much friction for the wheel to turn. Bend the wire so that it is perfectly vertical inside the tube/cap and only touching at the very tip throughout an entire turn of the pinwheel.
Make sure the unfolded part of the blades of the pinwheel are not sagging, and bend upwards until horizontal if they are. (The paper curls down a little from the initial heat of the candle.)
Adjust the angles of the folds in the blades, so that they are not too vertical and not too horizontal. (The angle with the incoming rising heat is important.)
If students are walking around the classroom a lot, the air currents they create will disturb the heat convection currents that make their pinwheel turn. Keep them seated as much as possible so that the candle flames are directly upwards.
Once students have made a candle pinwheel that works from these instructions, and they understand the need for the angled blades, they can make shapes of their own to test and troubleshoot.
As long as they have a large enough surface area of sloping blades and the card is balanced on the wire, it should turn. One student group made a heart shape that was a little unbalanced, but by moving the place that the tube was pushed through, and with with the help of my lighter at a critical point in its rotation, I could add enough heat to push it past its sticking point.
How it works:
Air heated by the candle flame flows upwards by convection.
The hot air rises and meets a sloping blade of the pinwheel, and pushes against it (the air molecules bang into the blades). Because of the slope of the blade, the blades are pushed sideways and because they are part of a wheel, they rotate.
Once everyone's pinwheels are turning, turn out the classroom lights for a beautiful scene.
To consolidate how it works, students can draw their pinwheel, showing how the heated air rises and hits the blades (blow out candles first).
Additional experimentation for students to try out:
Does it make a difference which direction the blades are folded relative to the incoming convection heat? i.e. can they angle up instead of down?
Does it make a difference how many blades there are?
Try a completely different shape: a flower or a starburst with many blades.
Good activity to follow this: candle chemistry.
Pinwheel shape idea from https://www.youtube.com/watch?v=_TRKsKMuYZ8