What was Miss Richards?

Hello, everyone. I’d like to tell you a story about Miss Richards.
   Miss Richards was a teacher at a school for boys and girls. She taught chemistry and physics from the lowest to the highest classes in the school. Sometimes the new classes learned rapidly, but sometimes they were very slow, and then Miss Richards had to repeat things many times.
   One year, the first class had been studying chemistry for several weeks when Miss Richards suddenly asked, "What is water7 Who knows? Hands up."
   There was silence for a few seconds, and Miss Richards felt sad but then one boy raised his hand. "Yes, Dick?" said Miss Richards encouragingly. He was not one of the brightest students in the class, so she was glad this boy could answer.
   "Water is a liquid which has no color until you wash your hands in it, then it turns black," the boy replied.