The first video shows Chaim Goodman Strauss leading a math circle exploring curvature via paper strips. Cutting triangles out of paper, then cutting the corners apart and placing the corners together is a start of this activity that would be suitable for first graders. The first graders could also place paper strips on surfaces and make polygons with funky angles from paper strips. The calculations with spherical triangles would be better appreciated with people in the 5th grader or higher. Hence the difficulty
matrix reflects the paper strip activity for lower grades, and the rest for 5th and higher grades.