Look at the screen you're reading this on. It's flat, right? It has a length and a width, but no real depth. Shapes that can exist on a flat surface like this are called two-dimensional, or 2D, shapes. Think of them as drawings or cutouts—perfectly flat.

While there are endless types of shapes, a few basics show up everywhere. You can identify them by counting their sides and corners. The proper term for a corner is a vertex.

Triangles are one of the most fundamental shapes. From a yield sign on the road to the pyramids in Egypt, their strong and stable structure is used all over the world.

Next up are shapes with four sides. You're probably familiar with these two:

So, a square is actually a special kind of rectangle—one where all the sides happen to be the same length. You can see this in floor tiles or a chessboard. Most doors and windows, on the other hand, are rectangles.

What about a shape with no straight sides at all? That would be a circle. A circle is a perfectly round shape. Every point on its edge is the same distance from the center. Think of a coin, a clock face, or a wheel.

As we add more sides, we get new shapes. Their names often give a clue as to how many sides they have.

Recognizing these shapes and their properties is the first step in understanding the world of geometry. Once you start looking, you'll see them everywhere.

Ready to test your knowledge? Let's see how well you can identify these 2D shapes.

Great work. Now you have a solid foundation for identifying the flat shapes that make up our world.