The version of that I learned in college is "How do you know God is a civil engineer?"

"...Because only a civil engineer would route a waste disposal system through a recreational area."

That, of course, it right up there with
"Chemical engineers build explosives.
"Mechanical engineers build bombs.
"Civil engineers build targets."

And, the classic:
"If it stinks, it's chemistry.
"If it's green and wiggles, it's biology.
"If it just sits there, it's physics."

Ok, done now....