In triangles $ABC$ and $DEF$,$\angle A = \angle D$,$\angle B = \angle E$ and $AB = EF$. Will the two triangles be congruent? Give reasons for your answer.

(NO) The two triangles are not necessarily congruent. For two triangles to be congruent by the $ASA$ (Angle-Side-Angle) criterion,the side must be included between the two angles. In $\triangle ABC$,the side included between $\angle A$ and $\angle B$ is $AB$. In $\triangle DEF$,the side included between $\angle D$ and $\angle E$ is $DE$. Since the given condition is $AB = EF$ instead of $AB = DE$,the triangles do not satisfy the $ASA$ congruence criterion. Therefore,they are not necessarily congruent.