Just as a plane is made of an infinite number of lines, a solid is made of an infinite number of planes that are right next to each other, like a stack of paper. A solid has width, and length, and thickness.If you looked at a solid from the side, it would look like a plane. Real objects like chairs and tables and snowballs and pieces of paper are solids.
If we just let a solid go on infinitely in all directions, it would take up the whole universe. So usually when we are talking about solids, we mean a specific part of the universe. We can define a solid, like a pyramid or a cube, by describing the planes on all sides of it, just the way we define a flat shape, like a triangle, by describing the lines on all sides of it.
Since we can put together lots of points to make a line, and lots of lines to make a plane, and lots of planes to make a solid, what would happen if we put lots of solids together? That is difficult for us to imagine, because in the world we live in that is not possible, but theoretically we can consider higher-dimensional objects that might result if we stacked up lots of solid universes. Or, if we think of objects as existing through time, we might consider that as a kind of stacking-up of universes: the universe of this second under of the universe of the next second, under the universe of the second after that, and so on.
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