Einstein's Twin is an NPC who can be found in the men's bathroom of The Club. He is there all day. He offers Einsteinium (which can be used to upgrade the Sledgehammer) to the player if his riddle is solved, which requires a minimum of 100 Intelligence. The player can alternatively fight him to obtain the Einsteinium, but this is not recommended, as he uses an Electro Helmet as his weapon. Einstein's Twin comes from Albert Einstein's twin paradox, which is an experiment on special relativity.
"Time... I can control it... speed it up... slow it down... when I speed up, time expands..." (if you do not have at least 100 Intelligence)
"This thing all things devours: Birds, beasts, trees, flowers; Gnaws iron, bites steel; Grinds hard stones to meal; Slays king, ruins town, And beats high mountain down." (if you have at least 100 Intelligence)
"Correct. A gift for such a clever fellow -- may it be of use to you." (after completing his riddle) (+5 Intelligence, Einsteinium)