My ex-brother-in-law lit his fart once (through his jeans) - was funny as fuck.
But, I think it would be better to drop the jeans, as when he did it, he got a 'blow-back' (probably because the gas couldn't escape from his jeans) -hehehehe
It burned a greenish colour (think it was the methane), and nearly set his clothes alight...

I never seen him move so fast, and watching a grown man slapping his groin (to put out flames) was just about enough to make me want to wet myself!

Has any one seen the film "Bio Dome"?
The blokes fart in each others faces, and the other one had to guess what food the other had eaten!
FAF