The Klein bottle is a closed non-orientable surface that cannot be embedded in 3D space without self-intersection. This is a figure-8 immersion — in true 4D, the surface never touches itself.
x = (a + cos(½ v) sin u − sin(½ v) sin 2u) cos v
y = (a + cos(½ v) sin u − sin(½ v) sin 2u) sin v
z = sin(½ v) sin u + cos(½ v) sin 2u
u, v ∈ [0, 2π], a = 3