1. The problem asks for a number that is both a square number and an even number. 2. A square number is a number that can be expressed as $n^2$ where $n$ is an integer. 3. An even number is any integer divisible by 2. 4. To find a square number that is even, we need $n^2$ to be divisible by 2. 5. This means $n$ itself must be even because the square of an odd number is odd. 6. For example, let $n=2$, then $n^2 = 2^2 = 4$. 7. The number 4 is both a square number and an even number. Final answer: $4$