Noticing different parts of buildings, like doors, can be an interesting exploration for children. It's also interesting to think about why certain kinds of doors are on certain buildings.
Remember the fascination of a revolving door when you were a child, or an automatic door that opened when you got close to it? Why do you suppose some big stores and other buildings have these kinds of doors? Imagine together what it would be like to have a door like that on your house or apartment.
If your child is really interested in doors and how they work, start to look more closely at how doors open and close. Look at the hinges and pins that hold household doors onto the frame. Can you figure out how they work?