Why do 3D engines primarily use triangles to draw surfaces?

Question

Why are triangles always used for drawing surfaces in 3D? Why not a square or some other shape?

Answer 1

Basically any complex (surface) structure can be represented as a bunch of triangles. The triangle is the most atomic and primitive geometry. Hence it is used as base for almost anything. Nevertheless most 3D engines provide you with more complex primitives like spheres, cones, cylinders, donuts, whatnot. Check your libraries documentation.