I heard this one from a teacher as well when I was very young! And it may well have been the same teacher telling us that blood was made of white blood cells and red blood cells, and I knew from my deep work in relevant fields (paints and crayons) that this combination did not result in blue.
I use to think the poles holding up traffic lights were hollow, and there was a person sitting inside throwing switches to change the lights while looking at a watch to keep the timing fair.