Several years ago in a high school Public Speaking class, a speech by one student involved passing a glass eye around the class. When the speaker asked if there were any questions, one of the students noticed that the glass eye wasn't transparent and said, "I don't understand. How do blind people see through this?"
Someone wrote to tell me about a whale watch he went on in high school. His teacher advised the students to bring a jacket in case it rained. This prompted one student to ask, "Well, where do the whales go when it rains?" The teacher just smiled and said, "They go under the water so they don't get wet."
"Oh, okay!" The student responded.