1. Know the base unit: The base unit in the metric system for length is the meter (m).
2. Establish the relationships: We need to know the relationship between kilometers (km), meters (m), and millimeters (mm).
* 1 kilometer (km) = 1000 meters (m)
* 1 meter (m) = 1000 millimeters (mm)
3. Perform the conversion: To go from kilometers to millimeters, we need to perform two steps:
* Kilometers to meters: Multiply the number of kilometers by 1000.
* Meters to millimeters: Multiply the result (in meters) by 1000 again.
Therefore, 1 kilometer = 1000 meters * 1000 millimeters/meter = 1,000,000 millimeters
So there are 1,000,000 millimeters in one kilometer.