YouTube user (and newly hired Khan Academy employee!) Vi Hart is really good at making math, science, and numbers seems interesting. I've written about her a couple of times and wanted to share the third and final installment of her series on spirals, the Fibonacci sequence, and the places they show up in nature. If you missed the first two, you can catch up with part one and part two.
Mold in the air conditioner: How serious is it?