Press Animate. Press Animate again to stop.

Drag points A, B, C, or D to change the shape of the resulting parabola. Then drag E and F to make sure point E is on C, and point F is on B. Click at the red 'X' before you animate again.

Press 'r' to return points to initial position.

Not all the initial positions of A, B, C and D will render a parabola. For example, if the two segments are parallel; or if the segments bisect each other.