When do students learn physics? How much, and in what way? Physics Education Research (PER) is the field of physics which studies how people learn physics and how to teach them better. A classic method in PER is to pre-test students before instruction, teach them, then post-test afterwards to see how much they've gained. However, this method cannot capture the dynamics of student learning. By testing students more frequently, we can observe rapid learning and forgetting, as well as destructive interference patterns. In this talk, I present data showing three kinds of "response curves" -- flat, step, and peak-and-decay -- and evidence that different demographics of students learn physics at different times and in different amounts.