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That's part of the surprise of a Japanese capsule machine. You don't know what is inside until you open it. Sometimes they're even a Pokémon. Red and Green established the way items are found but they never had a real concept of a modern pokeball until making the anime a year after the game launched. The pokeball in RBGY was just a two part thing that opened up. Some held pocket sized monsters, others items. By the time it reached international audiences they had hammered out most of the details.
