Bill threw a ball as hard as he could. It did not hit a wall, it was not attached to anything, and it was not caught and thrown back by another person. However, it came right back to him.

How is this possible?

brain boogies without square


credit: Fat Brain Toys

This is a very special brain boogie.

Above is an actual test question given to a second grader. As you can see, the student provided the penciled answer listed above.

Can you explain why his response is not necessarily wrong.