What causes a faucet aerator to get stuck?

A faucet aerator can be found in most homes, screwed onto the end of bathroom and kitchen faucets. These pieces add air to the water and screen out debris. They need to be cleaned often to keep them running, but sometimes can be difficult to remove from the end of the faucet. Faucet aerators are constantly exposed to water and various chemicals, which can gradually build up on the threads of the screw on aerators and make it difficult to remove them. In hard water areas, aerators are exposed to deposits such as lime, which can build up and cause an aerator to stick badly.