What are all these different dimensions about, anyway? Let's start at the beginning, with zero! Zero Dimensions: A point has zero dimensions. There's no length, height, width, or volume. Its only property is its location. You could have a collection of points, such as the endpoints of a line or the corners of a square, but it would still be a zero-dimensional object. One Dimension: Once you connect two points, you get a one-dimensional object — a line segment. A line segment has one dimension: length. Two Dimensions: A flat plane or shape is two-dimensional. Its two dimensions are length and width. Polygons, such as squares and rectangles, are examples of two-dimensional objects. Two-dimensional objects can be rotated in a plane. Three Dimensions: The objects around you — the ones you can pick up, touch, and move around — are three-dimensional. These shapes have a third dimension: depth. Cubes, prisms, pyramids, spheres, cones, and cylinders are all examples of three-dimensional objects. Three-dimensional objects can be rotated in space. Four Dimensions: What?! Just when you think you've got it all figured out, scientists start adding more dimensions. Most physicists — including Einstein — describe the fourth dimension as time. This fourth dimension is essential when studying an event. For example, imagine you are going to a party at the corner of 7th Avenue and 2nd Street on the 4th floor. You could abbreviate that location with the coordinates 7, 2, 4 — representing the three dimensions of space. But in order to describe an event, you need to add a time ­ or fourth coordinate. If the party is at 3 pm, your coordinates for the event would be 7, 2, 4, 3. So time is an essential dimension for discussing events.