A perfect square is a rational number whose square root is also a rational number.  In other words, there exists a number that when multiplied by itself gives you the perfect square.

An example of a perfect square is 9.  There exists a number, 3, that when squared gives us 9. 

The perfect squares between 1 and 100 are:

1, 4, 9, 16, 25, 36, 49, 64, 81, 100

You can create this list by squaring the first 10 numbers:

1², 2², 3², 4², 5², 6², 7², 8², 9², 10²

We can use our knowledge of perfect squares to help us estimate the square root of numbers that are not perfect squares.