A frustum. Specifically, a frustum of a triangular pyramid -- as one could also have a frustum of a cone, square pyramid, etc.
Read more: What_do_you_call_a_triangular_pyramid_with_the_top_cut_off
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square
It is a frustum of a rectangular pyramid.
A triangle
If you were to cut a cube in half along any plane passing through its center, you would get two equal halves of a cube. Each half would still have the shape of a cube with the same dimensions, just split into two separate pieces. The resulting solid figures would be two smaller cubes.
the humidity may have contributed to it somehow