If you start with a flap and add a little air and lengthen it, you get into trill territory. This is a good way to teach the trill. Once I recall I pronounced

If you start with a flap and add a little air and lengthen it, you get into trill territory. This is a good way to teach the trill. Once I recall I pronounced a drawn out /d/ when cold and startled, and a friend commented that I "rolled my D" and it was weird.