TETRAHEDRON • E-FREE TRANSLATION.COM • Definition of Tetrahedron

... methodical solutions, but not with the demonstrations of them. Like all other transitions in education, this should be made not formally but incidentally; and the relationship to constructive art should still be maintained. To make, out of cardboard, a tetrahedron like one given to him, is a problem which will interest the pupil and serve as a convenient starting-point. In attempting this, he finds it needful to draw four equilateral triangles arranged in special positions. Being unable ...

— Essays on Education and Kindred Subjects - Everyman's Library • Herbert Spencer