[28a top] R’ Yehuda in the name of Shmuel: One may not weigh meat on a scale against weights on YomTov [it is a prohibited act as being similar to a weekday activity]. Additionally, a person may not weigh the meat against a weight in his/her hands, if the person is an expert.
E’s Lesson: There is a real deep hidden message here in the Gemarah. An expert is viewed as a tool (like a scale); a tool of Hashem. When people need their cars fixed, bodies healed, or houses painted, Hashem sends His Messengers. We must always remember that whatever field we are in, we are only tools of G-d being used to fulfill a direct purpose.