Here's why:

* Ferromagnetism is a type of magnetism where a material exhibits a strong attraction to an external magnetic field. This is due to the alignment of the material's atomic magnetic moments.

* Iron, cobalt, and nickel have unpaired electrons in their atoms, which create magnetic dipoles. These dipoles can align themselves in a strong magnetic field, making the material strongly magnetic.

Let me know if you'd like to learn more about magnetism!