There's a tribe in the Amazon where the men 'invert' their penises, they turn it inside out and push it into their body, and then they only take it out when they need to pee.

I tried doing this myself but didn't have much luck. I watched a video online of some tribe guys doing it, and it looks like they had to apply considerable force the first time. I'm not risking bruising or 'fracturing' all those spongey tissues so maybe this one isn't for me.

It does seem like a cool idea though. A friend of mine can do it, and he does it to freak his girlfriend out.