Layla had sat out that last question due to the maneuvering she was doing in her seat to find the best way to stay in it. But this one was so obvious that she would feel stupid if she sat it out. "The Earth's tilt." Specifically, the way the Earth tilted on it's axis. Those saying that the seasons occurred because of the Earth's spin got a sigh from her. Smart people knew that made the days not the seasons. Duh.