Rotate the circle to unroll the circumference. You can also change the radius of the circle.

Pi is the ratio of the circumference of the circle to the diameter.
How many times does the radius "fit" into the circumference?
How many times does the diameter "fit" into the circumference?
what happens when you increase or decrease the radius?

The radian is an alternative unit for measuring angles. It is useful when studying circular motion as the length of an arc can be easily found from an angle in radians.
where s is length of an arc marked out by the angle and r is the radius of the circle.