On the ZX81, I knew from reading computer magazines that Sinclair BASIC had this weird single-key input method and it was mandatory. So, I managed to type in some very short program from the manual, SAVE it (I already knew how important that was!) and then RUN it. And it worked.
As this was more than my uncle had managed so far, in weeks or months or ownership, he was deeply impressed, and conveyed this to my parents, which as I type it very belatedly occurs to me is maybe why they bought me a ZX Spectrum the next year.
I don't think he ever used it much more. He was the son of Arthur Doodson, known for his work in tide prediction:
https://en.wikipedia.org/wiki/Arthur_Thomas_Doodson
Thom died 23 years ago. His wife was my auntie Valerie, a tiny bit famous in her own right for preserving Arthur's legacy. She remarried about 20 years later, and her husband David is still alive and still has that ZX81. I hope to get it off him some time.