Here's why:

* Milligrams to grams: There are 1000 milligrams (mg) in 1 gram (g). So, 5 mg is equal to 5/1000 g = 0.005 g.

* Scientific notation: Scientific notation expresses a number as a product of a number between 1 and 10 (the coefficient) and a power of 10. To write 0.005 in scientific notation, we move the decimal point three places to the right, giving us 5. Since we moved the decimal three places to the right, the power of 10 is -3.

Therefore, 5 milligrams is equivalent to 5 x 10⁻³ grams in scientific notation.