Mary has a magic carpetbag—not a magic carpet, like in Aladdin, though:

MICHAEL: That's a funny sort of bag.

MARY: Carpet.

MICHAEL: You mean to carry carpets in?

MARY: No. Made of.

When Mary first moves into the Bank's house, she pulls all of her things out of the bag, miraculously. Most of the objects she pulls out, like a lamp, are clearly too big to have ever fit in the bag—yet, there they are.

When Michael and Jane look in the bag, it seems to be empty, though Mary keeps taking things out of it. At one point, Michael goes under the table the bag is sitting on to see if this is some kind of trick. It isn't.

But, behind this, there's a moral lesson. Mary Poppins always knows how to sneak those in:

MARY: Never judge things by their appearance...even carpetbags. I'm sure I never do.

Isn't that nice? The whole movie's like that: Mary isn't just a kindly nanny. She's also practically a wizard. Bert's not just a charming chimney sweep: he's a magical guy, a Jack of All Trades, who lends support to Mary's adventures.

The idea is—don't take the world for granted, because it can surprise you with its hidden depths.