Sentence 1: A farmer was plowing his field when his plow hit something. 
 Sentence 3: When he started digging, he found that it was a big bone 
 Sentence 4:  A paleontologist came and excavated a mammoth's bone 
 Sentence 5:  The farmer donated the mammoth bone to a university museum 
 Option 1: The farmer dug and found a huge rock. 
 Option 2: The farmer thought he had hit a rock, so he needed to dig it out.