This is actually quite important - there's some objects you don't want to use, or things that you may think are ok but are not, and this is for safety. Here's a general list:
Things to not use:
Anything that you won't put a condom over
Fruits (you graduate, and you have to read some articles before using bananas
Anything sharp in any sense
Anything that will break (lightbulbs...yes I know, but I've heard stories)
Always use something smooth. Never use anything like the opposite end of the Sharpie.
An untrimmed fingernail
Using any of these might result in cuts (hard to tell if you're cut because it's hard to feel) and later infections that will need to get checked out by a doctor. If you do cut yourself, don't worry. Just wait a few days (2 at most). It should heal up pretty fast. If not, see a doctor!