A questioner said:
What is this very small doors/openings in old house? Any idea?

More info:
My MIL bought this house built in the early 1900s in Denver. On the first floor there are these two doors. One leads to the basement and the second leads outside from the kitchen. They are very small, about the height of a soda can.

Some of the answers:

1. My only guess was having an open house rat or bunny, or they wanted you to think they gave the house nice doors
2. Agree, it’s a cat door. Plenty big enough for many cats to slink through.
3. Doors for a model train set
4. I have seen these used as extension cord cut outs, with the door there to keep cold out. It is also useful for brining a hose through to get water out of or to something (like a cistern, well, something like that.) Not sure if that is the original purpose, but that is what they are used for now. Source: I also own a home from the early 1900’s
5. In Scandinavia we’re big on having little doors like that, for the elves. You know, Santa’s helpers. We will decorate them for December, to great pleasure for all the children. Then shut them up the rest of the year. They look a lot like this.Look up “nisse døre” on Google and you’ll see very similar results.
6. Cat door. Most cats can fit through, most dogs (and children) can’t.I’m actually considering installing one in my house, but it’s just a piano hinge that you put in the bottom of the door. You cut a bit off at a 45 degree angle, install the hinge between the rest of the door and the piece, and then you have a corner you can fold up when you want the cat to pass through, or fold down when you don’t. When I’m away, I have to block my dog from getting into my basement where my litter box is, so right now it means locking the cat in the basement.
7. Thomas Jefferson built holes in his upstairs doors so his cats could freely move around to switch vermin. Reminds me of that
8. It’s a cat door. My first house had them. Couldn’t try them out personally since I’m allergic to cats.
9. It might just be decorative for fun by a previous owner, making a cartoon-style mouse door with a little cute hatch.
10. As someone who recently moved out of an old house with these, yes, it’s a cat door. For those saying it’s too small, you either own a giant cat or you have never had cats. The only other alternative, pending the age of the house and which door it’s on, is that it’s a door for a hose to the basement. Several much older houses I’ve been in also had those.
11. I suspect they had an oil furnace at one time and the access was difficult for filling up the tank. Tank was in the basement and the shortest way was through the kitchen. Is the kitchen close to the road?
12. maybe a dust door sweep the floor, open the door and sweep it inI used the volunteer at a Victorian mansion/museum there was a door in the baseboard for this purpose on each floor. There was a little trash can in the basement to catch it at the bottom of the chute .
13. Here’s a similar thread I found from a few years ago that has some ideas.
14. I believe it was part of a vacuum system. They were popular back in the day. Capital building in Washington DC has them. You would hook a hose and wand attachment up the ductwork then turn it on. Duct work would be attached to the vacuum system which was probably in the basement of house and it would have a lid that clipped on the top of a metal barrel or cannister.
15. I think this set of doors, was so that a hose could go from the basement, out the kitchen door, and a pump could be used to de-flood the basement during spring thaw. With this setup, you could just have the whole thing ready and even running at night if it needs to, and you could still lock up.
16. It might be a mouse door. I was asked to build one into a baseboard once. I put a little stained glass in the door and motion sensing lights inside. Look up mouse house on pin you will see some examples

What do you think? Let us know in the comment!