Two guys are walking through the woods and come across this big deep hole. "Wow...that looks deep."
"Sure does... toss a few pebbles in there and see how deep it is." 
They pick up a few pebbles and throw them in and wait... no noise.
"Jeez. That's REALLY deep.. here, throw one of these great big rocks down there, that should make a noise." They pick up a football-sized rock and toss it into the hole and wait... and wait. Nothing.
They look at each other in amazement. One gets a determined look on his face and says, "Hey...over there in the weeds, there's a railroad tie. Help me carry it over here. When we toss THAT sucker in, it's GOTTA make some noise." The two men drag the heavy tie over to the hole and heave it in. Not a sound comes from the hole.
Suddenly, out of the nearby woods, a sheep appears, running like the wind. It rushed toward, then right past the two men, running as fast as it's legs would go then suddenly leaped in the air and straight down into the hole. The two men were astonished with what they'd just seen...
Out of the woods comes a farmer who spots the men and says, “Hey, you two guys seen my sheep out here?”
“You bet we did! Craziest thing we ever seen! It came running like crazy and just jumped into this hole!”
“Nah”, says the farmer, “That couldn't have been MY sheep. My sheep was chained to a railroad tie.”