I used to believe that the 2nd serve in tennis was slower than the 1st because you were still knackered from putting all your energy into the 1st!
A childhood belief about sport

others believed things like this
- I forgot the expression "not cricket", so I said, "not tennis" instead.Anon
- I used to believe that all my energy "came from my nipples and my back" I literally said that to my parents.— Eleanor R
- I Used To Believe The Monster Beverage/Energy Drink Was Monster Milk!— KellGirl
- I thought it was against the law to carry around a lot of money unless you were putting it in the bank.Anon
- I used to believe that you could catch a bird by putting salt on it's tail— Eli
- I used to believe that putting lemon juice in yours eyes could lighten them.Anon