A circle is <u> always </u> similar to another circle, because we can <u> always </u> map one onto the other using only dilations and rigid transformations.
Put another way: we can stretch or compress any circle, and move it to another location, so that it matches up perfectly with any other circle. You can use similar triangles to help with the proof of this if you wanted. Right triangles might be the easiest to work with.
