A childhood belief about general

As a boy, during school dinners, I really used to enjoy the turnip that was regularly served with our meals.
All the other kids hated the stuff, but I couldn't get enough of it, and regularly used to finish off their unwanted portions.
For some reason I ended up telling my mother about this, and - because she didn't like the thought of me eating off other kids' plates - she told me that turnip was well-known for stunting the growth, and that schools were encouraged to serve it by cunning parents, so that kids wouldn't outgrow their school uniforms too quickly.
This certainly put me off the vegetable very quickly, and for years afterwards I believed this to be gospel, and would casually inform other people about the dangers of this growth-stunting root vegetable.
It wasn't until I was about twenty - and my height had stayed stubbornly at five feet no inches - that I casually observed to my mother that perhaps I'd been warned too late about the turnip after all. It was then that she shamefully admitted her deceit, and I enjoyed my first serving of turnip in years!