There is this girl named Shelly who always turned in her homework late. When she had a report due today her professor asked her why she didn't have it. She told him this story: ''I was eating lunch yesterday when my dog went berserk. I didn't know why. Then I saw it. There was a squirrel on the tree. I saw it go up but then slowly back down and run away. Then my dog chased after it so I left my report on the table. When I came back my report had disappeared.''. When the professor heard this he knew she was lying. How did he know?