Number of non-square numbers between ${11}^2$ and ${12}^2$ is .

All natural numbers are not perfect squares.

A perfect square can never be expressed as the product of pairs of equal prime factors.

A number having $2, 3, 7$ or $8$ at its unit place is never a square number.

A number having $0, 1, 4, 5, 6$ or $9$ at its unit place is always a square number.

A number ending in an even number of zeros is always a perfect square.

Square of an odd number is always an odd number.

$1, 3, 6, 10, 15$, ...... are called triangular numbers.