A shadow of a shadow of a tesseract/hypercube/fourth-dimensional cube.
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What is a tesseract? A tesseract is a cube in the fourth spacial dimension. It has straight edges and 90 degree angles like all squares/cubes.
Dimension zero is a point. I don't mean a dot, -- I mean a point. A point has no height, width, or depth. It's like taking one of our shadows and turning it onto its side, and then squashing that depthless line down to its own depth.
The first dimension is dimension zero piled on top of itself over and over again: A line. A line has height, but no width or depth. It is like taking one of our shadows and turning it onto its side.
The second dimension has height and width, but no depth. It is the first dimension piled over and over again. Our shadows are in the second dimension. We see in the second dimension.
The third dimension is the second dimension piled over and over again. It is the dimension we live in.
The fourth dimension is the third dimension piled over and over again. Its shadows would be in three dimensions.
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This is a shadow of a shadow because the fourth dimension has three dimensional shadows; a shadow of something three dimensional is two dimensional, as depicted here.