A 12-year-old boy got lost on the subway after going out alone on Sunday. The boy came to Shanghai to visit his parents during the summer holiday. Although his parents were happy to see him, they ____ little time with him. The boy did his homework for two ____ and felt bored, so he went out to play on the subway. He had fun playing there. But when he felt tired and hungry, and wanted to go home, he didn't know at which station he should get off. The police found the boy ____ on the subway and took him to the office. They bought him something to eat and asked him some questions. The boy's parents couldn't find him after arriving home. They felt ____ and called the police right away. They got to the police office and saw ____ son. They were very happy and thanked the police.

笑笑家最近买了一个微波炉，他发现，当微波炉工作时，家里的电能表转得飞快，他想了想最近学过的物理知识，明白了其中的道理是（    ）