Perfect Square
A natural number is called a perfect square if it is the square of a natural number. Perfect squares are also called square numbers.
For example, the numbers 1, 4, 9, 16, 25 are perfect squares because
12= 1 = 1 × 1
22= 4 = 2 × 2
32= 9 = 3 × 3
42= 16 = 4 × 4
52= 25 = 5 × 5
However, numbers such as 6 and 10 can not be written as square of natural numbers.
6 = 2 x 3 and 10 = 2 x 5