How can you get an irrational number?

Answer 1

For example, by taking the square root of any positive integer, except a perfect square. Thus, the square root of 2, 3, 5, 6, 7, 8, 10, 11, etc. are all irrational.

You can also make up a rule to write a number in decimal, that does not involve a regular repetition. Note that, for example, 5.4871313131313... (repeating "13" forever after that) is rational. However, if you write, for example, 0.1010010001000010000010000001... (adding one more zero each time) will give you an irrational number, since all rational numbers will repeat the same sequence of digits over and over eventually.