Miss Richards was a teacher at a school for boys and girls. She   __1 chemistry and physics from the lowest to the highest classes in the   __2  . Sometimes the new classes   3   rapidly, but sometimes they were very   4  , and then Miss Richards had to   5   things many times.

One year, the first class had been studying chemistry for several   __6_ when Miss Richards suddenly asked, "What is water? Who knows?   7__   up?"