Let V be a vector space over a field F and let V1, V2 be subspaces of V . Prove that the union of V1, V2 is a subspace of V if and only if

Question

Let V be a vector space over a field F and let V1, V2 be subspaces of V . Prove that the union of V1, V2 is a subspace of V if and only if

V1⊆V2 or V2⊆V1 .

Please someone explain why this holds?