Here's a breakdown:
* Grass: Poison is the most common weakness of Grass types, as it represents a natural counter to plants.
* Poison: While it may seem counterintuitive, Poison-type moves are super effective against other Poison-types. This is because many Poison-type Pokemon have vulnerabilities to their own type.
Important Note: While Poison is super effective against Poison-types, it is also not very effective against Ghost, Rock, Ground, and Steel types.