This is a hands-on activity for students studying geometry. The sheet has 2 triangles on it and a place to record the measures of the interior angles of each one. Students practice working with a protractor, measuring accurately, comparing results with their group, and discussing human error. Eventually, they should see that the sum of the interior angles of a triangle is always 180. Students should be pushed to see why this must be true, instead of taking it for granted, and coming up with the reasoning there will help them function as a class and develop their critical thinking and problem solving skills.