I used to pull pranks on my sister (21 months younger than I) all the time. Hey, she’s blonde, it’s like shooting fish in a barrel! One day (she was maybe about 16 at the time) I had her convinced that people from Pennsylvania are called “pencils”. She initially thought I was pulling her leg (something about me saying stuff like that before to her or something like that, I don’t know) but after a while started to believe me. She was still a little skeptical when my dad, usually the authoritarian father figure, walked by her room and casually remarked, in a straight face, “he’s right, they are called pencils”. Well, now she was convinced!
This was one of the very rare instances I remember (read: only one) that my mom called both my dad and I to her to discuss our behaviour.
And “pencils” remains a family in-joke 20 years later.