### Theory:

A natural number is a perfect square if it is the square of other natural numbers.
Example:
$\begin{array}{l}36=\sqrt{36}=6×6={6}^{2}\\ \\ 49=\sqrt{49}=7×7={7}^{2}\\ \\ 25=\sqrt{25}=5×5={5}^{2}\end{array}$

Therefore here the $$36$$, $$49$$, and $$25$$ are the perfect squares because these numbers are obtained, from squared by the exact whole numbers.
Not perfect squares:

If the square number ends with $$2$$, $$3$$, $$7$$ and $$8$$ will not be the perfect square.
Example:
Consider the numbers $$12, 23, 47, 88$$ and take the square root; the result will not be the whole numbers.

$\begin{array}{l}\sqrt{12}=3.464\phantom{\rule{0.147em}{0ex}}×3.464\phantom{\rule{0.147em}{0ex}}={\left(3.464\right)}^{2}\\ \\ \sqrt{23}=4.796×4.796\phantom{\rule{0.147em}{0ex}}={\left(4.796\right)}^{2}\\ \\ \sqrt{47}=6.856×6.856\phantom{\rule{0.147em}{0ex}}=\phantom{\rule{0.147em}{0ex}}{\left(6.856\right)}^{2}\\ \\ \sqrt{88}=9.381×9.381\phantom{\rule{0.147em}{0ex}}={\left(9.381\right)}^{2}\end{array}$
From the above two explanations, you can see the difference that, perfect squares $$36, 49$$ and $$25$$ will be the square of whole numbers, but the other square numbers will not be the square of whole numbers.