[Note, in order to fully understand this post, you should first listen to the following portion of a Dave Attell stand-up routine. It's kind of long, and while I recommend you listen to all of it (after all, he's a funny man), skipping to the eight and a half minute mark gets you to the relevant joke.]
A few weeks ago at the gym
Friend: If a guy just walked by me in the locker room totally naked, then a few minutes later passed by again with his penis flapping around, does that mean I needed to get out of there?
Me: Absolutely. Wait, was he running?
Friend: No. The first time he passed me he was walking, but the second time he was kind of doing this jog/shuffle thing. But his penis was definitely flapping. Did that mean something bad was coming from that direction?
Me: Not if he wasn't running full speed. In that case you just need to be wary, and pay attention to your surroundings. If he's running full speed? That's when you need to worry. If/when that happens, don't even grab anything, except your car keys if they're handy, just get out of there.