Okay, another funny thing from last year.
We have a very small laundry room that you step through coming into the house from the garage. I wanted to cut a pet door next to the door from the living room so we could put the litter box in the laundry room and the cats would always have access to it. That way the expended litter could be taken straight to the trash thing in the garage without being carried through the house. And there's a utility sink in there, to which we added hand soap and a towel rack so we could wash our hands after dealing with the litter.
Here's a picture, which I actually took because of Bonnie's habit of sitting in that doorway when I'm doing laundry, watching the washing machine spin.
R cut that door, and put the frame on without the flap, under Bonnie's interested observation. When he finished she walked over, sniffed all around it, went through and came out, and said "cool" so to speak.
When Molly walked in and saw it - whoa, different story! A hole had opened in the universe. Maybe it was a wormhole into dimension X. If you stepped through it, you could end up anywhere; or maybe it would snap shut when you were halfway through, and there you'd be. So even though Molly would happily walk back and forth through the open door you see in the picture, it took several days to get her used to the idea that she could go through that small opening safely. I had to go into the laundry room and shut the door, and play with her through the pet door, and take Bonnie in with me, so Molly could look at her in there and see her jump out. Eventually Molly screwed her courage to the sticking place, and pulled up her socks, and tried it herself. And nothing bad happened, so we were able to put the litter box in there and it worked out like we thought.
Molly now likes to go in there and glare out through the pet door. It's kind of startling to be randomly looking around and happen to notice her green eyes staring out.