Here's why:

* Different units: Milligrams measure mass, while pounds measure weight.

* Weight vs. Mass: Weight is the force of gravity acting on an object's mass. Mass is the amount of matter in an object.

To make a conversion, you need to know:

* What the substance is: Different substances have different densities, meaning the same mass can occupy different volumes.

* The context: Are you looking at the weight of a substance on Earth? On a different planet?

If you're trying to figure out the weight of something in pounds, you need to know the substance's density and the gravitational force acting on it.