What's the dumbest thing you thought as an adult that you recently learned was wrong?

I just realized while cooking that a measuring-cup cup (as measured out as 250mL in a glass measuring cup) is the same amount(s) as one of the actual plastic baking measuring cups that go inside each other like Russian dolls lol

I thought they were different somehow (something something imperial metric yadda yadda yaddda)

Your turn to come clean Lemmings!

**EDIT: to clarify, I mean volumetrically for measuring liquids