I believed the right word was administration. Unfortunately, the correct term was menstruation.
A childhood belief about periods

others believed things like this
- When I was you younger, I used to believe men had their menstruation.— Lilly-Rose Bessin-Collin
- I used to believe that menstruation fluid was blue because of the way TV commercials showed it. — matias
- I believed that wind was caused because the trees moved. I was completely sure my conclusion was correct.— Glenr
- i used to think that the word "stupid" was a curse word, ranging in scale right beside the "f" word.Anon
- i believed the f-word was fart until i was about 8.— Kendra
- my husband believed, until recently, that the word "several" meant seven of something. — sheryl