After you've taken pictures of all six Pokémon signs, Prof. Oak will open a path to the Rainbow Cloud, where you can take a picture of the rarest Pokémon of them all, Mew. It will take time to get a good shot of Mew, you'll need to use all the skills you learned to get a picture that will impress Prof. Oak. When Mew first appears in the blue shell, hit it three times with some Pokémon Food or a Pester Ball. Mew will then appear in a yellow shell that you'll have to do the same to, but this time it will be a lot harder because Mew doesn't stay in front of you. After the yellow shell comes off, Mew will appear in the open heading toward her shell again. Hit Mew with some Pokémon Food or a Pester ball to make her spin. Keep hitting her until she's in the middle of the path and you get pretty close. Then start snapping pictures and hopefully you'll get a huge one.