Stress and strain are fundamental concepts in material mechanics. Stress (σ) is the internal resistive force per unit area when an external force is applied to a body, given by $\sigma =\frac{F}{A}$. Strain (ε) is the resulting deformation or change in dimension per unit length, $\epsilon =\frac{\Delta L}{L}$. The external force causes stress, and this stress then induces strain in the material. They are directly related by Hooke's Law for elastic materials: $\sigma =E\epsilon$, where E is Young's modulus.

Final Answer: The strain in a body is caused by stress.