English: Central part of sphere eversion (turning inside out) process flattened with folds along a square, cut out of doubly colored paper (red/green). The depicted part is homeomorphic to a cylinder/collar/bottleneck, to be glued to two spheres opposite oriented. The middle stage is the Morin surface, with both sides equally privileged. The left stage can be easily unfolded to a green (outer side) sphere, the right - red. The middle stage has removed four flat triangles to see the folds underneath, the triangles can be shifted above of beneath the surface to proceed to the left/right. Note the quadruple intersection in the center, double intersections along diagonals and fold annihilation at the vertices of the inner square (midpoints of sides of the outer square).

English: Central part of sphere eversion (turning inside out) process flattened with folds along a square, cut out of doubly colored paper (red/green). The depicted part is homeomorphic to a cylinder/collar/bottleneck, to be glued to two spheres opposite oriented. The middle stage is the Morin surface, with both sides equally privileged. The left stage can be easily unfolded to a green (outer side) sphere, the right - red. The middle stage has removed four flat triangles to see the folds underneath, the triangles can be shifted above of beneath the surface to proceed to the left/right. Note the quadruple intersection in the center, double intersections along diagonals and fold annihilation at the vertices of the inner square (midpoints of sides of the outer square).