Memorial Pillar

Battle all the people on the strip of land, and surf south a bit more. You'll reach a larger island with a huge stone in the middle and a Metal Coat on the left. The huge rock is the grave of the trainer's Onix, Tectonix. If you leave a Lemonade there, the trainer will give you TM42 - Facade.

Now that everything's taken care of, go to One Island and hand Celio the Sapphire gem. With that, the machine is fixed, and now Pokemon from Ruby and Sapphire can be traded to FireRed and LeafGreen. In addition to that, you'll be able to fight the Elite Four again, and will also be able to catch Mewtwo in the Cerulean Cave in Cerulean City. If you fight the Elite Four again, they'll have stronger Pokemon, and you can do that on your own time. Now, it's time to catch the strongest Pokemom: Mewtwo!