Gretchen ran out to the store, leaving Ruby out of her kennel downstairs. She's not yet housebroken, but the first floor is reasonably well puppy-proofed, so nothing too bad could happen.
Turns out Ruby has now figured out how to climb the stairs to the second floor, as I discovered when a very excited puppy showed up in my office. After she investigated the entire second floor, I picked her up, carried her back downstairs, gave her a (declined) chance to potty outside, and put her back in her kennel.
I feel guilty about this, but the second floor is nothing resembling puppy-proofed.