To use clay solids and dental floss to derive your answers, mold a cube from the clay. Then use the dental floss or piano wire to make a straight slice through the cube, creating a cross section. When making a cut, you may end up with "bent" cuts, but what you want to try to make is a planar slice. For example, think about slicing straight through a loaf of bread.
Alternatively, you may want to use colored water in plastic solids. Fill the plastic solids part way, and then tilt them at different angles to see the faces of cross sections created by an imaginary slice along the surface of the water.
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