This lesson is designed to get students thinking and communicating about triangles. In the Warm-up, students use a Notice and Wonder activity to activate prior knowledge about triangles. Then students make sense of and solve (MP1) a problem about side lengths that will and won't form a triangle. Using strips of graph paper, they explore different side lengths to understand that not all sets of three lengths can form a triangle. Students then pair up to share ideas (MP3) and come to a conclusion about sets like 4, 6, 10 and 4, 6, 2. While ideas about the Triangle Inequality Theorem may come up, neither a formal nor informal statement of the theorem is expected from students.