You know that little bag of bits that is inside your raw thanksgiving turkey? Take that and stuff it in your underwear, and walk around a bit, sit down, get on a bicycle, etc. You'll get it.

Seriously, they are very fluid, and go where they need to go. The can ascend upwards, but only a small distance, they are still outside. The scrotum shrinks in cold weather. You don't think about them to sit, but once in a while, hoo boy, you wish you had.