Pokemon Did You Knows?

[retrolinkx]

In the original Pokémon Red, Blue, Green and I think Yellow, once the player receives the Pokédex from Prof. Oak and then goes on to beat their rival without buying a Pokéball, nor evolving their starter, the player can receive 5 Pokéballs from Prof. Oak telling him that you need to capture Pokémon to get them in your Pokédex.

Source

Also In Gold and Silver, if the player evolves their starter before they get a Pokédex the game believes you've already got it due to the fact you shouldn't have a second Pokémon since you don't have an Pokéballs yet, and you only get them after you get the Pokédex.