I wouldnt recommend it, but air is probably the safest way to go if you want to give it a try out of curiousity. The first time I tried it with an electric air pump it felt amazing.
So much so that I made a habbit out of doing it (im a raging glutton) after about the eighth time I did it the pain started to outweigh the pleasure. I asked my doctor about why I kept getting cramps increasing in intensity and admitted to my fetish and he said it had something to do with the muscles in my colon becoming overdeveloped and wasnt anything serious.
It was just really painful, so much for gradually increasing my capacity. I havent done it since, though I could probably get away with it once in a blue moon. I just wanted to tell my little story so no one makes the same mistake I did.
Just out of curiousity.
How does one go about inflating ones self?
Beauty is in the eye of the beholder.