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Reversible Figures
When 2d drawings are used to represent 3d objects, it is possible to create ambiguous figures that appear to "pop" in and out. The classic example is this line drawing of a cube:
There are two ways to view this cube. Look for the two squares is the picture (there are many trapezoids and a couple rectangles, but only two squares). One of the squares should appear to be the front of the cube, and the other should appear to be the back of the cube. Now, imagine that the back face is now pushed to the front, and the front face is now pushed to the back. The 2d drawing can represent either of these cubes.
If we remove some of the lines, it becomes perfectly clear which two cubes could have been represented by the 2d drawing above.