I'm here to tell you the story of a robot named Carl. He came from a far away land known as Factory. Carl was sad because he was missing a part called a tire. He also needed a sun gatherer. But, the tire was more important. Once Carl got all these parts he could travel to his new home in the nation of Lab and the city of Office. It was a tricky thing to get there with missing parts. Just as he had given up hope Carl got a message from Mr. X saying the new parts were ready to be delivered. This made the robot very happy. The parts arrived a few days later and Carl put them in with 2 days of work. After this Carl began to travel the last bit of his goal to get to his new job. After this Carl took 10 days to get to Lab. 
Which was the most important missing part that Carl has to get after getting out of the factory?
A round tire