Who was 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 learnt rapidly, but sometimes they were very slow, and then Miss Richards had to repeat the things many times.
   One year, the first class had been studying chemistry for several weeks then Miss Richards suddenly asked, "What is water? Who knows? Hold up your hand."
   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 that 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 with great confidence.