Largest eigenvalue of matrix and the largest eigenvalue of its submatrix

Question

Why the largest eigenvalue of matrix is more than or equal to the largest eigenvalue of its submatrix?

If $A$ is $n \times n$ symetric matrix and $A'$ is $n-1 \times n-1$ its submatrix why is true $\lambda_{max}(A)\ge \lambda_{max}(A')$? I don't know how to figure it out.