Prospective elementary teachers at three universities engaged in online modules called the Virtual Field Experience, created by the Math Forum. The prospective teachers learned about problem solving and mentoring elementary students in composing solutions and explanations to nonroutine challenge problems. Finally, through an asynchronous online environment, the prospective teachers mentored elementary students. The researchers assessed the prospective teachers' solutions and explanations to problems at the beginning of the semester, at the middle of the semester after completing the training in mentoring, and again at the end of the semester after the mentoring was completed. The researchers observed improvements in the prospective teachers' abilities to write explanations to problems. Specifically, growth was seen in prospective teachers' communication of their explanations and their ability to construct viable arguments and critique the reasoning of others (Common Core State Standards Initiative, 2010, Standard for Mathematical Practice 3), and attend to precision (Standard for Mathematical Practice 6).