A father took his sons to an amusement park one day. He told them to meet him at the entrance at 5:00 later that day. When the kids finally showed up at the end of the day, it was 5:30. The father said that he was angry and wouldn't take them again for a whole month. One of the boys said, "Wait, it wasn't our fault! We were on a roller coaster, but as we went down the first hill the engine blew out!" After thinking for a while, he grounded the boys. How did he know they were lying?