Final answer:
All circles are similar because their corresponding parts are proportional.
Step-by-step explanation:
To prove that all circles are similar, we need to show that their corresponding parts are proportional. One way to do this is by comparing their radii. Since the radius of a circle determines its size, if we have two circles with radii 'r' and 's', and 's' is twice as long as 'r', then all corresponding parts of the larger circle will be twice the length of the corresponding parts of the smaller circle. This means that all circles are similar.