Kid Questions: What Makes Willy-willies?

We used to get lots of willy willies where we grew up too, but I didn’t know then what caused them. Do you ever do kid question suggestions from others? DD asked the other day why a mirror reflects your picture but you can see through glass. I showed her with some aluminium foil behind glass to try and show her how a mirror worked. No idea what makes the aluminium/silver/other things reflective though…