Some Pokemon evolve when they are happy enough; all the babies, togenic, eevee, ext. Get them happy by giving them items like carbos, not letting them faint, getting hair cuts, level up, steps, and keeping them out of the PC.
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You can go to Veilstone City and take your Chansey for a massage there. When its happiness is full, it will evolve by its ownself. Hope I helped.
Level it up when it its happiness is full
I think with full happiness (then level it up one level)It needs to have max Happiness AND you need to have the National Dex.
Go to the Pokemon cente in eterna and talk to the lady with brown hair and she will give you the happiness app (the one with all your Pokemon floating around and bumping into each other). Hold your stylus on apokemon of your choice and if 2 big hearts come out of it, its happiness is near if not full.
in cerulean city in a house next to the PC get pikachu to full happiness and then talk to her