In the world of Pokemon, there are 18 different types. Since Generation I, there is Normal, Fire, Fighting, Water, Flying, Grass, Poison, Electric, Ground, Psychic, Rock, Ice, Bug, Dragon, and Ghost. In Generation II, Dark and Steel were introduced. And in Generation VI, Fairy was available.
Now, with all of these types, there are numerous combinations of one type being strong against another, one type being weak against another, one type being resistant to another, and one type being vulnerable to another. However, many make no sense. (Probably for balancing purposes.)
10 Normal And Fighting
Normal is strong against nothing, weak against Rock, Ghost, and Steel, resistant to Ghost, and vulnerable to Fighting. And Fighting is strong against Normal, Rock, Steel, Ice, and Dark, weak against Flying, Posion, Psychic, Bug, Ghost, and Fairy, resistant to Rock, Bug, and Dark, and vulnerable to Flying, Psychic, and Fairy.
Why do Fighting-types have the advantage over Normal-types? Fighting-type Pokemon are able to beat up tough opponents of Rock, Steel, and Ice (and Dark, for some reason). Normal-type Pokemon are plain and regular — not being good versus any other types — yet a punch or a kick takes them out.
9 Flying And Rock
Flying is strong against Fighting, Bug, and Grass, weak against Rock, Steel, and Electric, resistant to Fighting, Ground, Bug, and Grass, and vulnerable to Rock, Electric, and Ice. And Rock is strong against Flying, Bug, Fire, and Ice, weak against Fighting, Ground, and Steel, resistant to Normal, Flying, Poison, and Fire, and vulnerable to Fighting, Ground, Steel, Water, and Grass.
The only thing that comes to mind when analyzing why Rock-type Pokemon are good versus Flying-type Pokemon is the phrase, "Kill two birds with one stone." Sure, a stone would obliterate a poor bird, but that Pokemon must have serious aim to make contact in the first place. Flying-types are up in the sky, not on the ground like the other disadvantaged types (Bug, Fire, and Ice) when it comes to Rock-types.
8 Bug And Grass
Bug is strong against Grass, Psychic, and Dark, weak against Fighting, Flying, Poison, Ghost, Steel, Fire, and Fairy, resistant to Fighting, Ground, and Grass, and vulnerable to Flying, Rock, and Fire. And Grass is strong against Ground, Rock, and Water, weak against Flying, Poison, Bug, Steel, Fire, Grass, and Dragon, resistant to Ground, Water, Grass, and Electric, and vulnerable to Flying, Poison, Bug, Fire, and Ice.
Bug-type Pokemon taking advantage of Grass-type Pokemon is weird. The two types should live harmoniously together. Then again, the other Pokemon that are bad versus Bug-types makes no sense either: Psychic and Dark.
7 Steel And Electric
Steel is strong against Rock, Ice, and Fairy, weak against Steel, Fire, Water, and Electric, resistant to Normal, Flying, Poison, Rock, Bug, Steel, Grass, Psychic, Ice, Dragon, and Fairy, and vulnerable to Fighting, Ground, and Fire. And Electric is strong against Flying and Water, weak against Ground, Grass, Electric, and Dragon, resistant to Flying, Steel, and Electric, and vulnerable to Ground.
Electric-types somehow have the advantage of Flying and Water but not Steel? Steel-types are only not very effective versus Electric-type Pokemon. When considering how metal conducts electricity, it really feels like Electric-types should have total dominance over Steel-types.
6 Water And Ice
Water is strong against Ground, Rock, and Fire, weak against Water, Grass, and Dragon, resistant to Steel, Fire, Water, and Ice, and vulnerable to Grass and Electric. And Ice is strong against Flying, Ground, Grass, and Dragon, weak against Steel, Fire, Water, and Ice, resistant to Ice, and vulnerable to Fighting, Rock, Steel, and Fire.
It is just odd that Water resists Ice, and not even neutral. That seems backward. Ice-type Pokemon should be super effective against Water-type Pokemon since, you know, cold enough temperatures will completely freeze over water (rendering it useless.)
5 Fairy And Fire
Fairy is strong against Fighting, Dragon, and Dark, weak against Poison, Steel, and Fire, resistant to Fighting, Bug, Dragon, and Dark, and vulnerable to Poison and Steel. And Fire is strong against Bug, Steel, Grass, and Ice, weak against Rock, Fire, Water, and Dragon, resistant to Bug, Steel, Fire, Grass, Fairy and Ice, and vulnerable to Ground, Rock, and Water.
Fairy-types suffer when battling Poison-, Steel-, and...Fire-types? What is it about Fire-type Pokemon that gives them protection against Fairy-type Pokemon? Fairy-types can conquer literal Dragons (some known for breathing fire) but have trouble dealing with some flames?
4 Psychic And Dark
Psychic is strong against Fighting and Poison, weak against Steel, Psychic, and Dark, resistant to Fighting and Psychic, and vulnerable to Bug, Ghost, and Dark. And Dark is strong against Ghost and Psychic, weak against Fighting, Dark, and Fairy, resistant to Ghost, Psychic, and Dark, and vulnerable to Fighting, Bug, and Fairy.
This one here is just bizarre. The reasoning behind Dark-types having the advantage over Psychic-types is absent. Maybe the mind games Psychic-type Pokemon play on their opponents fall short or get completely reversed when dealing with the evil Dark-type Pokemon.
3 Ground And Poison
Ground is strong against Poison, Rock, Steel, Fire, and Electric, weak against Flying, Bug, and Grass, resistant to Poison, Rock, and Electric, and vulnerable to Water, Grass, and Ice. And Poison is strong against Grass and Fairy, weak against Poison, Ground, Rock, Ghost, and Steel, resistant to Fighting, Poison, Grass, and Fairy, and vulnerable to Ground and Psychic.
Why? Ground-types have the advantage over Rock and Steel because like earthquakes and Fire and Electric because the soil contains flames and electricity. However, Poison-types are full of like acid and chemicals which have no problem contaminating the earth and seeping through.
2 Ghost And Ghost
Ghost is strong against Ghost and Psychic, weak against Normal and Dark, resistant to Normal, Fighting, Poison, and Bug, and vulnerable to Ghost and Dark.
Ghost-types get the advantage over other Ghost-types. Are ghosts scared of ghosts? Where is the logic here?
1 Dragon And Dragon
Dragon is strong against Dragon, weak against Steel and Fairy, resistant to Fire, Water, Grass, and Electric, and vulnerable to Ice, Dragon, and Fairy.
Dragon-types are good versus other Dragon-types. How is one fearsome creature better than another fearsome creature? Monsters eat monsters in the Pokemon world, it seems.