Consider the two buildings A and B in the following figure.

Suppose you are standing on one side of the road and these two buildings are on the other side. Can you tell which building is taller?

Yes, you are right. Simply by looking at the buildings, you can say that the building on the left (i.e. building A) is taller. This is called the method of observation, where we look at two objects and say that which of them is taller or which is shorter.

Now, let us apply this concept of observation in the field of geometry. You already know what a line segment is. Remember that the length of a line segment is the distance between its end points.

Now consider the line segments AB and CD...