A fresh co-pilot is flying with

A fresh co-pilot is flying with a seasoned airline captain. The captain says to the co-pilot, "I want to test how well you know the airplane."

"I am going to go to the lavatory for a #2", says the captain. "When I come back, tell me whether the airplane gets lighter as I relieve myself."

The captain went to do his business, and when he comes back, he asks the co-pilot: "Well, son, is the aircraft any lighter now"?

The co-pilot eagerly answers, wanting to impress the captain: "No, sir, our aircraft does not release lavatory waste when in flight. It is securely stored in the onboard septic tank until the airplane lands and it is flushed by the maintenance crew. Therefore, as a closed system, the weight of the aircraft remained the same before and after you relieved yourself."

"All you think about is shit, son!", says the captain. "The airplane is lighter now because I was away for 15 minutes and we used up two hundred gallons of jet fuel during this time!"