So, this is one way to get MEW...
u start out in Cerulean City. u must not have beat the gym there and must not have battled the guy volgende to the nugget bridge that says,"I saw your feat from the grass!"and of course u must go and defeat the nugget bridge people. With all that said, lets get to how the cheat works! u go up route 24 and as soon as u get to the end of the nugget bridge head left. Go down until u are one step away from being directly volgende to the water that takes u to the Unknown Dungeon. As soon as u walk that one step press START right when u press the down button. The guy standing there will see u but instead of engaging in battle the START menu will come up. Then u will have to teleport (using an ABRA u can catch from the route connecting Cerulean and Saffron City.) Use ABRA to warp back to Cerulean. (you must have healed at the pokemon center there first.) The START and "A" button will not work. Go and battle the swimmer at the gym and after u beat him, the buttons will be able to work again. Head north and right when u reach route 24, the START menu will pop up. When u close the menu, u will go into a battle (without being in the grass! WOW!) and the pokemon u will battle is a MEW! Now here is the hardest part. Catching this pokemon is not easy. It is treated like a legendary so u have to either put it to sleep of paralyze it and knock out most of it's HP. As soon as u do that, u will have yourself a MEW!
