The sequence of numbers are all perfect squares, meaning that their square roots are whole numbers.
The square root of 4 is 2.
The square root of 9 is 3.
The square root of 16 is 4.
The square root of 25 is 5.
So, we need to find what number, when square rooted would equal 6.
This number equals 6 * 6 or 36.
The next number in the sequence would be 36.