What process can demagnetize a steel nail?

Heating a steel nail in a flame can successfully demagnetize it due to the effect of temperature on the magnetic properties of materials. When steel is heated to a certain temperature, known as the Curie temperature, the thermal energy causes the magnetic domains within the steel to become disordered. In their normal state, the magnetic domains are aligned in a way that gives the nail its magnetism. However, when heated, they begin to vibrate more intensely and can lose that alignment, ultimately leading to the loss of magnetism once cooled.

This process contrasts with other options presented. Rubbing the nail with a plastic object typically does not generate enough energy to disrupt the aligned domains significantly. Submerging the nail in water does not affect its magnetic properties at all, as water does not interact with the magnetic domains. Exposing it to a strong magnetic field could actually realign the magnetic domains in the same direction, potentially increasing its magnetism rather than demagnetizing it. Thus, the method of heating the nail effectively disrupts its magnetic structure, making it the correct choice for demagnetization.