What really IS a real number?
//Search many definitions of what a real number is.
http://www.jamesbrennan.org/algebra/numbers/real_number_system.htm
http://en.wikipedia.org/wiki/Real_number
http://www.mathsisfun.com/numbers/real-numbers.html
http://mathworld.wolfram.com/RealNumber.html
http://www.math.vanderbilt.edu/~schectex/courses/thereals/
What is NOT a real number?
http://www.mathsisfun.com/numbers/imaginary-numbers.html
http://en.wikipedia.org/wiki/Imaginary_number
http://mathworld.wolfram.com/ImaginaryNumber.html
//list and compare the differences.