This chapter presents a detailed model of children's explanations of where the sun goes at night. Knowledge of the day/night cycle is one of the first relatively complex sets of knowledge that all people acquire. The model shows how children progress through a lattice of possible explanations (a lattice is a partially but not completely ordered set). The task and data modeled offer an excellent basis for the investigation of order effects, with implications for modeling scientific discovery and for learning in general. It shows that some transitions are particularly difficult, that some transitions require using incomplete or incorrect knowledge, and that not all transitions are possible. It also shows that the order of learning can make a large difference in the amount that has to be learned and, perhaps more importantly, unlearned. Better orders provide about a 30% reduction in facts that have to be learned. These findings make suggestions about the instructional complexity that children, and presumably learners in general, can handle, and about the use and importance of intermediate stages of learning.