Here’s one from “The Nephew Files”:
What is the difference between the gas we pass by burping and the gas by pass by farting? You can ignite a fart, but you can’t ignite a burp, what are the differences between the two? Thanks: DJ (Drollman’s Nephew!)
I think that a burp is usually air. Read that somewhere.
Peace,
mangeorge
Cecil has covered this indirectly: http://www.straightdope.com/classics/a1_079.html
Note that two of the gasses that make up a fart are flamable. And this is probably where mangeorge read about belches being mostly air.