Here's how to convert centimeters to nanometers:

* Know the conversion: 1 centimeter (cm) = 10,000,000 nanometers (nm)

* Set up a proportion: 25 cm / 1 cm = x nm / 10,000,000 nm

* Solve for x: x = 25 cm * 10,000,000 nm/cm = 250,000,000 nm

Therefore, 25 centimeters is equal to 250,000,000 nanometers.