Any irrational number never ends. So, you can take square roots. 4 is the square root of 16, and 6 is the square root of 36. So, take the square root of any number between 16 and 36 except for 25. For example, 17 is between 16 and 36. So, sqrt(17) is therefore irrational. You can't take the square root of 25 because that would result in a rational number, 5.