In this video the instructor explains the concepts of rational and irrational numbers. Multiplying a number by itself gives the value of its square. If you apply the square root to that squared number it returns to you the original number. This square root is also called a radical. A square root of a positive number can have two values. For example the square root of 81 is either 9 or -9; this is because when you multiply 9 with itself the square is 81 and even when you multiply -9 with itself the square is still 81. Real numbers consist of rational numbers and irrational numbers. Rational numbers are any numbers that can be written as the ratio of two integers a/b where b is not equal to zero. The rest of the real numbers are irrational numbers. This video explains the rational and irrational numbers.