In days of old,
when knights were bold,
and toilet paper wasn't invented,

you'd wipe your ass
with a blade of grass
and walk away contented.

This one's more of a practical joke. The person telling it uses a paper napkin as a prop.

"So you're out on a hike and you have to take a shit. All you've got to wipe with is one napkin (pull out the napkin). How do you do it?"

Let the listener mull over the dillema for a little bit, then demonstrate.

You fold the napkin in half twice, and then rip off the corner. "Hold on to this, its very important. Do. not. lose this." Then you unfold the napkin, stick your middle finger through the middle-finger sized hole, and just fucking scoop that whole shit out in one motion. Then you grab the napkin around the underside and carefully pull it off the finger, twisting back and forth, wiping the finger clean as you pull it off.

Give the listener the chance to ask, "well what do you do with the corner?" but don't wait too long. Pull it out, and use it to clean out under your fingernail.