The other day, my friends and I had a discussion about the most useful invention of all time. There were many good idea- the train, the car, and the airplane. They're all useful, but they were not my choices. For an everyday, useful invention, I vote for the air conditioner. I live in Taipei, and during the summer the temperature can be 35 degrees Celsius or higher. It's so hot! I'm not joking when I say that, without an air conditioner, people here couldn't work or study. The weather is so hot that it can make you feel unwell if you don't drink enough water and get enough rest. What do you guys think is the most useful invention of all time?