This lesson plan introduces students to irrational numbers through an activity where they observe and group examples. The teacher presents examples like 1.5, 3, 5, 3 and asks students to group them in two stages - first by variables and numbers, then by variables, rational and irrational. Students learn that numbers that cannot be expressed as a ratio of two integers, like 1.5 and √2, are called irrational numbers. The lesson concludes with students practicing identifying irrational numbers in examples.