That the moon has no gravity. And that’s exactly what I was told before the moonlanding, and children’s videos etc. are still saying it 40 years after the moonlandings. I’ve never been able to understand why teachers and children’s videos can’t say the moon has less gravity than earth. They might not understand a fraction like 1/6, but any child can understand “more” or “less,” as any child will prove when calling our attention to that they got “more” or “less” of something than their siblings.
Also, my daughter was told that people in Australia are upside down. The truth is that it was a shaila put to HaRov Tzvi Pesach Frank as to how the lulav should be held during Succos when in the Southern Hemisphere.