What was making the robot curve to the left?

The regional robotics contest in Fort Worth was two days away, and the robot that Lucy and Jamal had built for the Riverside Middle School team still would not drive straight.

"It's the code," Jamal said for the third time that afternoon. He set the robot at one end of the practice mat and pressed start. It rolled forward, then slowly curved to the left until it bumped into the wall. "See? Every single time."

Lucy had written the program, and she had already checked it line by line twice. Both wheels were told to spin at exactly the same speed. Still, she opened the laptop and read through it again, because arguing with Jamal felt like arguing with the wind.

When she finished, she had not found a single mistake. She closed the laptop and picked up the robot instead. She turned it upside down and spun the left wheel with her finger. It wobbled. Then she spun the right wheel. It turned smoothly and quietly.

"Jamal," she said, "come feel this."

He pressed his thumb against the left wheel and moved it back and forth. A small screw that held the wheel to the motor had worked itself loose. Each time the wheel turned, it slipped a little, so that side of the robot fell behind.

Jamal was quiet for a moment. Then he rubbed the back of his neck. "I put that wheel on," he said sheepishly. "I was so sure it was the code that I never even looked."

He tightened the screw, and they tested the robot again. This time it rolled all the way across the mat in a perfectly straight line.

On Saturday, before every match, Jamal checked each screw on the robot himself. Lucy did not have to remind him once.