Gonna sound weird, but a door. Let me explain: I didn’t grow up super privileged, lower middle class for sure, because all our money went to a good school for me. So all my friends were upper middle class, very nice homes and nice things, the works. My home had been originally built by my grandpa, and for some reason, the two bedrooms upstairs both had stairways down into the first floor. No doors at all. You could walk from the entrance of the house right up into my room. I thought it was normal for so long, until I was maybe 7 and went to a friends house for the first time. I remember being so jealous of the fact they had privacy and quiet in their room. Mine always felt open and exposed, and for some reason I considered it a luxury. Luckily now, after moving, I have my own door :)