How was Sophie able to venture into Howl's past? Why did that black door appear?

hikariyagami said:
that black door is the portal used by howl to get into different places. it is also the door grandma sophie used to get into the castle. since howl uses it to get into different places, we can assume that it is magical as the whole castle itself. we can also say that is a portal to get into his heart, since his heart is with calcifer who manages the entire castle. sophie has the ring given by howl that tells where the castle with his heart is, and i think that is how she is able to travel into howl's past.

siggney said:
the best answer is magic. in the book the black door actually led to howl's hometown (modern day Wales) but in the present time, and there's this whole funny segment of Sophie trying to describe what a car is.