As you know all objects are made up of atoms and atoms are made up of electron,protons and neutrons . electron orbits the nucleus but electron have a spin when it is orbiting like our earth around the sun. According to physics electron forms small magnetic field due to its spin .as electron orbits the nucleus in differnt direction the magnetic field gets cancels out that is the reason why all objects expect magnet are not magnetic.but in magnets the case is different , electron orbits the nucleus in the same direction. The magnetic field of each electron adds up and forms a strong magnetic
field. When we magnetise a material like iron with a magnet the electrons of iron start orbiting the nucleus the the same direction and hence form a magnetic field but for sometime.after sometime it's electron starts orbiting in the different direction and the fields cancels out
Can you guess why plastics cannot be magnetized?