PokéBase - Pokémon Q&A
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So I was trying to play through Pokémon Blue version and I had a Staryu on my team. It had just reached level 36 and then, it started evolving! I swear I didn’t use a Water Stone on it. Can anyone tell me what’s going on? And also, if it’s worth it to keep cancelling evolution until Hydro-Pump, or just teach it Surf?

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Did the battle involve an Onix?
As for the part about the water attack, I think you should teach it surf. It has more PP and can be used to traverse water.
It was at the saffron city gym. The psychic type one. And yes I do have an Onix on my team, why?

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In Red and Blue, if a Pokémon that evolves with an evolution stone levels up, and another Pokémon with the same index number as the required evolution stone is on the field when the battle ends, the stone-evolving Pokémon will evolve after the battle. Onix shares its index number with the Water Stone, so winning the battle with Onix after Staryu leveled up caused Staryu to evolve.

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What is the source that you found this information from?
This is really cool!