My assumption is that friction between two rocks threw off a spark that ignited pocket lint. An alternate theory is that the whole thing is a lie to cover up some really stupid thing the two of them did when they got home.
Did something like that once myself. I think I was 10, however, and chained it to a metal pole. Not quite sure how the other kid felt when he found out his dad stole my bike to give to him.
To bother them and bite them.
Little fleas have lesser fleas
And so on adinfinitum."
http://www.khanacademy.org/#vi-hart
Come to the Dark SIde - XL