The progression of the taxonomic organization of life from Linnaeus's original two kingdoms to the traditional five-kingdom system to today's widely accepted three-domain system is explored in a group-learning activity. Working with a set of organisms, students organize them into each system. Discussion after each step focuses on viewing classification schemes as hypotheses about the relatedness of organisms and how hypotheses are altered with accumulation of new data. Finally, the connection between phylogenetic trees and the hierarchal system of biological classification is emphasized by using tree-thinking to analyze the universal phylogenetic tree as the basis of the three-domain system.