FACTUAL RIDDLES XXXVIII

A 16-year old boy's room was always a mess. His mother told him that he couldn't go out with friends until he had cleaned at least half of his room. A few minutes later he was heading for the front door. His room looked exactly the same, but he had satisfied his mother's demand. What did he say to his mother as he left to meet his friends?

Answer:

"I cleaned the top half."