Sampling is a fundamental practice of many scientific disciplines. However, K-12 students are rarely asked to think critically about sampling decisions. Because of this, open questions remain about how best to support students in this practice. This study explores the emergent sampling practice of two classes of sixth-grade students as they investigate the ecology of a local creek. It draws on student interviews, pre/post-tests, student artifacts, and video recordings of classroom activity to identify and trace shifts in the ways in which students approached collecting data. The findings suggest three ways in which students' attention to variation within the context of their ecological investigations supported their development of a more sophisticated practice of sampling.