Professor: Many people use a microwave oven every day. How many of you know that the microwave oven was the result of an accident? During World War II, scientists invented the magnetron, ( ) a kind of electronic tube that produces microwaves. We’re all familiar with microwave ovens, but ( ) a microwave? Well, it’s a very short electromagnetic wave. Anyway, in 1946, an engineer named Dr. Percy Spencer was standing close to a magnetron he was testing. He suddenly noticed something unusual. He felt something warm in his shirt pocket. He reached in and discovered that the candy bar in his pocket was a hot, chocolaty mess. ( ) , the candy bar had melted. Dr. Spencer was so excited because he realized that microwaves could raise the internal temperature of food. ( ) , microwaves were able to cook food from the inside out! And do it very quickly. Dr. Spencer saw the possibilities here. His next step was to build a metal box into which he fed microwave power that couldn’t escape. He put various foods inside the metal box and tested cooking them. In time, he invented something that would revolutionize cooking—the ubiquitous microwave oven. By that ( ) that we see microwave ovens just about everywhere.