Chapter 13, Problem 13.85E

In propene ( ${\text{CH}}_{\text{3}}-\text{CH}={\text{CH}}_{\text{2}}$), the first carbon has $s{p}^3$ hybrid orbitals and the second carbon has $s{p}^2$ hybrid orbitals. These orbitals interact to make a $\sigma$ bond. Why are these hybrid orbitals not orthogonal?