You want to convert 1 Kilogram to Milligrams. You already know that 1 kilogram =1,000 grams. Explain how knowing that 1 gram =1,000 Milligrams can help you .
one kilogram would equal to 1000000 milligrams. you multiply the two conversions. Basically, you are splitting each gram that is in a kilogram into 1000 pieces.
