Cooperative learning groups will discover the definitions of rational and irrational numbers using this card sort activity. By creating their own categories to sort a variety of numbers including terminating and non-terminating decimals, fractions, and square roots, students will explore many mathematical concepts and have opportunities to review their prior knowledge regarding fractions and decimals. Some will notice perfect squares, and many will identify the difference between rational and irrational numbers through experience rather than definitions. This resource is an adaptation of Introduction to Rational and Irrational Numbers Card Sort by Kyla Hohnhorst and is used under a CC BY 4.0 license.