Magnetism is a force which is generated by the motion of electrons within atoms. Electrons orbit the nucelus of atoms and some metals, such as iron are ferromagnetic. Magnetic fields are generated due to their magnetic moment, and these fields interact with other things that also have magnetic moment.

Electrons have a property known as quantum mechanical spin. This creates what is known as a magnetic dipole moment. In non magnetic materials, the moments in the material are in a random arrangement, often pointing in opposite directions and cancelling out the magnetic fields they generate. With magnetic materials, particles have spin in a particular direction, resulting in magnetism, and the magnetic fields are cumulative.

With metals, such as iron, there are unpaired electrons and they spin in the same direction, and the field they generate is cumulative, so you get a metal that is attracted to or repelled by magnets.

Elements can be changed into other elements through nuclear reactions, but this wouldn’t be possible through more simple means. If you ate a chunk of iron for instance, it would remain as iron. Rather than being a solid piece of iron it’d be fragmented and dispersed throughout the body so it depends on how sensitive your tools are for detecting magnetism.