I’m teaching a class on mechanics. Analytical mechanics. Torqued, not shaken. (That’s covered in, I shit you not, a class on vibrations.) I’m at the board, giving a lecture.
“Now, as you can see, if we have a shaft with non-constant cross sectional area, we know that the polar moment of inertia varies as a function of X. So let’s say we take a slice out of that shaft. A pancake, of thickness dx. What does that mean?”
After a long silence, a student: “The polar moment of inertia…”
Me: “WRONG. What does it mean to have a pancake of thickness dx?”
After another long silence: “The maximum shear stress will be…”
Me: “WRONG. A PANCAKE OF THICKNESS dx IS A CREPE.”