1. The problem is to write the number $8.6 \times 10^{-3}$ as an ordinary decimal number. 2. Recall that $10^{-3}$ means $\frac{1}{10^3} = \frac{1}{1000}$. 3. So, multiply $8.6$ by $\frac{1}{1000}$. 4. $8.6 \times \frac{1}{1000} = \frac{8.6}{1000} = 0.0086$. 5. Therefore, $8.6 \times 10^{-3}$ written as an ordinary number is $0.0086$.