When a cube is sliced by a plane that contains the midpoint of three edges that meet at one vertex, the resulting cross-section is a triangle.
To visualize this, consider a cube with vertices labeled. Take one vertex (let's call it \(A\)) and identify the three edges that meet at this vertex. By slicing through the midpoints of these three edges, the plane will intersect the cube in such a way that it connects the midpoints of the edges, effectively forming a triangle.
Therefore, the shape of the cross-section formed by slicing the cube in this manner is a triangle.