As a – self proclaim – clear descent into geekdom, I always crack on this joke. It reminds me to when I first met “pi” in junior math and square root -1 in high school.
A brief mathemetical review: a rational number is one that can be expressed as an integer divided by another integer (i.e., a/b). Well, Mathematics is not for the lesser mortals. And negative square root, they called imaginary numbers, because positive x positive and negative x negative will results in positive number.