Story about two siblings who run away from home and stay in a museum at night

Question

I remember when I was a kid my dad read us a book about two children, brother and sister, who ran away from home, and stayed the night in a museum. They slept under a bed (that was an example of a certain style of furniture, if I remember right), because nobody ever cleaned under there.

I'm having a hard time remembering the plot, but I remember the brother telling about how he played War with his friend, and when they had to leave they would take their cards with them, and spit on the cards and say "Thou shalt not shuffle". Later in the story, the brother said he got his money by cheating in this game.

And at one point in the story, they rent a PO box for a quarter, because they can't get it for just two days.

I hope these details are correct; it was years ago when I heard the story. Does anyone have any idea what the title and author of this book could be?

Answer 1

This is From the Mixed-Up Files of Mrs. Basil E. Frankweiler, by E. L. Konigsburg.

The brother and sister in question are Jamie and Claudia Kincaid, who run away from home. They live in the Metropolitan Museum.

Jamie did both of the things that you describe with the cards:

Jamie wanted to sit with his buddy, Bruce. They played cards on the bus; each day meant a continuation of the day before. (The game was nothing very complicated, Saxonberg. Nothing terribly refined. They played war, [...]

And:

Every night when Bruce got off at his stop, he'd take his stack of cards home with him. Jamie would do the same. They always took a vow not to shuffle. At the stop before Bruce's house, they would stop playing, wrap a rubber band around each pile, hold the stack under each other's chin and spit on each other's deck saying, "Thou shalt not shuffle." Then each tapped his deck and put it in his pocket.

And about the PO box:

"Here's the plan: we rent a post office box in Grand Central. Like when you send in box tops, you always send them to P.O. Box Number So-and-so.

We write a letter and tell them to answer us at the box number. After they tell us that they need help, we reveal ourselves. As heroes."

And the quarterly part:

"For how long?" the man inquired. "For about two days." "Sorry," the man said, "we rent them quarterly." "All right, then. I'll take eight quarterlies. That makes two days." "Quarter of a year," the man said. "That makes three months."

Regarding the bed:

At last she found a bed that she considered perfectly wonderful, and she told Jamie that they would spend the night there. The bed had a tall canopy, supported by an ornately carved headboard at one end and by two gigantic posts at the other.

Lastly, you also noted about laundromats in the comments. They are mentioned 4 times, here's one:

They packed their gray-looking laundry back into their pockets and walked to the door of the laundromat. At the door Claudia turned to Jamie and asked, "Can we ...?"