This article from The Science Teacher magazine describes a unit of study on natural selection. Students begin by trying to explain the phenomenon of the exponential increase in a population of fish. The activities in the lessons include "Oh Deer" to teach about limiting factors and selective advantage, observations of sunflower seeds to explore variation and a "Wormeater" game for students to experience natural selection. These activities guide students as they develop a model of natural selection and then use that model to construct Darwinian explanations about how a population changes over time. The science practices are incorporated into the lessons . The article is written for high school level students, but the same approach has been used in the middle school classroom with similar results.