CD + Microwave = Fire

Don't try this at home.

Why it happens:
The microwaves cause current to flow through the thin aluminum layer of the CD (as they would in any aluminum or metal in a microwave - don't try that at home either). The heat from the electricity causes the aluminum to vaporize. The electricity forms arcs through the vapor that continue to form new trails on the CD. Eventually the electric fields build up in hot spots at the edges of these trails. The intense heat at these hot spots causes the polycarbonate of the CD to combust, resulting in a flame.