Why do CDs crack in a microwave oven?

CDs crack in a microwave oven due to the presence of metal layers within their structure. When microwaved, these metal layers can produce sparks and generate heat, causing the plastic material of the CD to warp and crack. The uneven heating and rapid expansion can lead to structural failure, resulting in visible damage to the disc. Additionally, the microwave's electromagnetic waves can disrupt the integrity of the CD's data layers.