You can move the points A, B, C and D by clicking them and dragging them with your mouse.
By doing this you'll see two important rules about angles inside circles being enacted. Look carefully at the size of the angles. Try to work out the rule before you click the button "Show Theorem" at the bottom.

You will flip from one rule to the other as the lines on the diagram cross over each other. Try it.
Enjoy yourself while you practise and learn these two important theorems by name.