What's funny about math class?

Tell a joke that non-math majors can't get to the point: the boss gave engineers, physicists and mathematicians a bucket of paint to cover the walls separately. After a while, the boss walked into the engineer's house and found that only one wall was painted. The engineer said, I didn't find enough paint until I finished painting! The boss walked into the physicist's house again and found nothing painted. The physicist said, I calculated and found that there was not enough paint, so I didn't apply it. Finally, the boss walked into the mathematician's room and found that the wall was covered with paint, and there was no shortage of paint in the bucket! The boss was surprised and asked the mathematician what was going on. The mathematician replied: very simple, I only drew all the rational points!