Here's why:

* Magnetic fields are not blocked by non-magnetic materials. Cardboard is made of paper, which is primarily composed of cellulose, a non-magnetic material.

* Magnetic fields can penetrate materials. Magnetic force lines can travel through a variety of materials, including non-magnetic ones like cardboard. The strength of the magnetic field might be slightly reduced as it passes through, but it will still be present on the other side.

Analogy: Imagine a light shining through a piece of paper. The paper might make the light slightly dimmer, but the light still passes through. It's similar with magnetic fields and cardboard.

However:

* Magnetic force can be affected by ferromagnetic materials. If the cardboard had a metal object embedded in it, like a metal staple, the magnetic force might be attracted or repelled by that object.

Let me know if you have any other questions!