To guide Santa on is way we light up the sky with the Northern lights

This demonstration shows the displacement of copper from copper chloride by aluminium (which is higher in the reactivity series) leaving solid copper and aluminium chloride. The copper can be seen to form around the aluminium.

The acid removes the aluminium oxide layer so the reaction can take place; this will also react with the aluminium to produce hydrogen gas. This can be ignited to produce green/blue flames.