Step 1: Verify the Congruent principle.

           NOTE: Two circles are congruent if they have the same size and shape.

          The size can be measured as the radius, diameter or circumference.

          They can overlap.

          Two circles are congruent if they have the same diameter

          So, All circle are not congruent.

Step 2: Check the similarity principle

          DEFINITION:  Two figures are similar means that they share a common shape. They can be different sizes, but they must have the same shape. For example, observe this image of the butterflies:

Similar Figures 1e

So, Any two circles are similar because they have the common shape but they are in different sizes.