Here's why:

* Nano means one billionth (10⁻⁹).

* Milli means one thousandth (10⁻³).

Therefore, there are:

* 10⁻³ seconds in 1 millisecond

* 10⁻⁹ seconds in 1 nanosecond

To find the number of nanoseconds in a millisecond, we divide the number of seconds in a millisecond by the number of seconds in a nanosecond:

(10⁻³) / (10⁻⁹) = 10⁶ = 1,000,000