A cube, "cut a knife" through three points A, B and C, and cut off a corner of the cube; After points A, B and D, "cut a knife" and cut off a "corner" of the cube; Cut a "corner" from the cube by passing through three points B, C and D with a knife; Cut off the fourth "corner" of the cube by crossing the three points A, C and D with a knife;

Observe the geometry surrounded by four parts. What geometry is it? How many sides does it have? What are the characteristics of each surface?

Answer:

It is surrounded by an equilateral triangle vertebral body, which has four faces, each of which is an equilateral triangle.