I recall once in 6th grade we were talking about marine sponges and the teacher had brought in one of those loofah sponges, passed it around and said that it was a sponge from the ocean. I raised my hand and said "no, that's a loofah. It's from a plant that's in the cucumber family. I know because my grandmother grows them." And he was like "No, this is a sponge from the ocean. Your grandmother can't grow these." There was no changing his mind and he didn't appreciate being contradicted.