Angles in Circles
Click the "Show Angles" box to show the size of angles CBA and FED.
Then, unclick the "Show Angles" box and slide the Rotate bar to rotate segments CB and FE. Guess what the new angle measures are and click the "Show Angles" box to see how close your prediction was to the actual measure.
Try this a few times and see what you notice and what questions you think of.