The Professor's Nightmare (also know as Equal Unequal Ropes) is the title of the classic rope magic trick, whereby three ropes of totally different lengths, all transform into equal lengths and then back again into unequal pieces.
It's given the title of Professor's Nightmare from the routine that's supplied with it, whereby a story about a professor is told to demonstrate the magic happening with the ropes.
The full effect is the magician shows three pieces of rope, one long, one medium and one short. The pieces are taken by their ends between the magician's hands and at the appropriate moment, pulled apart to show they have transformed into pieces of equal length. Then with another gesture, the magician causes them to return to their original lengths.
A super piece of magic, what's more, it's easy to perform! You're supplied with all that's needed including online tuition in how to perform the full routine, plus how to change all three pieces into one long piece as a climax and how to perform the Cut & Restored Rope trick.