How to level up quickly in 'Pokémon Go' without spending money

Being "the very best" means different things to different people. Some people might want to fill up their Pokédex as quickly as possible, for instance, while others may want to be the strongest and cruise around town conquering gyms.
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The thing is, the two sort of go hand-in-hand in Pokémon Go. Catching 'em all and being a gym-annihilating badass both come down to one key thing: your level.It's no c