Two bats, Joe and Mark, are going for their midnight feed.
After an hour or so, Mark gets tired of looking and goes home with no blood.
Joe comes home hours later with blood dripping from its mouth.
Mark says enviously, "Where did you get all that blood from?"
Joe replies, "Follow me. I'll show you."
After awhile Joe leads them to a cave. He says, "You see that wall over there?"
The hungry bat Mark excitedly says, "Yes!"
Joe says, "I didn't."
