Look, less of the attitude please, I thought this was supposed to be one of the most friendly forums on the internet! I'm just trying to point out to somebody starting out in using CDs as a way of storing data that from a technical point of view, its not as simple as just writing or printing away on the surface of your CD however you like as you can end up with damaged data further down the line (will probably start off as harmless skipping and spinning in your drive).
As you have unreasonably in my opinion failed to see the point of testing something for yourself, if you'd like to read
http://www.cdrfaq.org/faq07.html#S7-7-1 this will give you some basic info on the surface structure of a CD and will explain how and why pens can damage them. TDK CDs used to come with a label saying not to write on them with anything other than an oil-based pen. Others say to use an alcohol-based pen, I'd say just don't write on them at all, or just write on the circle before the data layer begins if you really have to.
As I have stated, its not worth writing on a CD/DVD/etc simply because pen inks and CD construction techniques change over time and you simply don't know how one is going to affect the other. We are effectively talking about chemicals reacting with each other over time and it is difficult to predict how this will happen under different conditions (for example, some CDs degrade in light and others, in paradox, in lack of light, according to the materials used).
Some people, as above have never noticed any of this as they will have probably used a good quality pen with a good quality CD marking layer on the CD and pressed lightly, etc, and so will simply not be aware that this is not always the case with all CDs.
Please note, the above link only considers CDs - DVDs and BlueRay discs, etc will be different again, as the manufacturing process will differ accordingly.