Instead of the knife you can also use a sanding block and fine sandpaper.
Be aware that this method is quite secure,but also produces a lot of dirt, so it is a good idea to destroy the cd in a box or something similar and not on the carpet. A hoover/vacuum cleaner also might be a good idea
A quick way of destroying a cd:
1.A cd consists of 33000 sectors, but if you destroy sector 16 it will take anyone alot of effort to read the cd, due to sector 16 consists of the “booting information”. Just take a knife or firelighter(be aware that the gases are quire unhealthy!),… and destroy the first milimeters around the hole in the middle of the cd! This method ist not really secure if you want to protect your data against powerful enemies (NSA?, people like that,…)
2.A really quick way: Take all the cds you want to destroy and put them into the microwave, start the microwave and wait a few seconds until the structure of the cds beginns to change,…, wait a few seconds longer and turn off the microwave. All data should be destroyed,but also you should never again cook your meal in that microwave!