Here's why:

* Magnitude refers to the size or amount of something. In physics, when referring to a body, it usually means its mass.

* Force is an interaction that can change the motion of a body. This means it can cause acceleration (a change in velocity), but it doesn't change the body's fundamental properties like its mass.

Think of it this way:

* Imagine pushing a shopping cart. You can apply force to make it move faster or slower, but you can't change the cart's weight (its mass).

* Similarly, a rocket engine provides a powerful force to launch a spacecraft, but it doesn't change the spacecraft's mass.

However, force can change the body's energy:

* Force can do work on a body, which changes its kinetic energy (energy of motion).

* Force can also change the potential energy of a body, like lifting it against gravity.

In summary: Force can change a body's motion, energy, and even its shape, but it cannot change its fundamental property of mass (magnitude).