^ I thought so too the last time I posted this pic for somebody. I just posted it since it was the best Google images had to offer. Then somebody else came up with a good point. Having a stem in a gravity bong will pull the smoke through the water filtering and cooling it like a regular water bong. You're right, I don't think it's necessary, but after that was brought to my attention, I'd vote for having a stem as long as it's not too much trouble to make one. Always something to be learned on this site, even for the pro smokers.