Since the turn of the 20th century, psychologists have tried to pin down what it means to be smart. More than 100 years later, they're still arguing about the best methods to spot intelligence. But that squishiness hasn't stopped organizations from NASA to Mensa from adapting psych assessments into some of the trickiest exams ever to meet the No. 2 pencil. Ready to give 'em a go? Time starts... now.