I can identify an irrational number.

Numbers that are not rational are called irrational numbers.

[Real Numbers classification diagram: Rational Numbers and Irrational Numbers are distinct subsets of Real Numbers. Within Rational Numbers are nested Integers, Whole Numbers, and Natural Numbers.]

Rational Numbers examples: -4/5, 0.75, 31.8

Integers examples: -5, -16/4, -1,000

Whole Numbers example: 0

Natural Numbers examples: 19, √4

Irrational Numbers examples: √2, 1.121121112..., π, -√3

The number 2 is not a perfect square, so √2 is irrational.

The number 4 is a perfect square, so √4 = 2 is rational.