A maths professor, during their lecture, says "obviously, we now have equation 42" when a student interrupts, asking "how is that obvious?" The professor looks

A maths professor, during their lecture, says "obviously, we now have equation 42" when a student interrupts, asking "how is that obvious?" The professor looks back at the blackboard, starts to speak, but then remains silent as a consternated look falls across their face. After ten minutes of silent pondering, they erase three blackboards and manically fills them with equations, derivations, and other expressions. After another half-hour of furious scribbling--eventually filling both sides of two more free-standing chalkboards--they exclaim, "AHA! It is obvious!"