Case Study 1: Breath of the Wild --- The Chemistry Engine That Changed Open-World Design

A World Made of Materials, Not Scripts

On March 3, 2017, The Legend of Zelda: Breath of the Wild launched alongside the Nintendo Switch, and it quietly dismantled thirty years of open-world game design convention.

Open-world games before Breath of the Wild were content libraries. They were filled with hand-placed quests, scripted encounters, collectible markers, and authored activities --- hundreds of individually designed experiences scattered across a map. Skyrim, The Witcher 3, Grand Theft Auto V, and Assassin's Creed all followed this template: a large world populated with designer-made things to do. The world itself was scenery. A beautiful, detailed backdrop for the real content: the quests, the dialogue, the scripted set pieces.

Breath of the Wild inverted this. The world was not a backdrop. The world was the content. Every object in the game was a material with physical and chemical properties. Every material interacted with every other material according to consistent, universal rules. The result was a game where the most memorable moments were not authored by the designers. They were discovered by the players.

The system that made this possible has no official name. Fans and developers call it the chemistry engine --- a set of material interactions that treat Hyrule not as a game level but as a simulated chemistry lab.

How the Chemistry Engine Works

At GDC 2017, director Hidemaro Fujibayashi and technical director Takuhiro Dohta presented a talk titled "Change and Constant: Breaking Conventions with The Legend of Zelda: Breath of the Wild." In it, they revealed the architecture of the chemistry engine.

The system is built on two layers:

Layer 1: Material Properties

Every object in the game has a material type. Material types have properties:

This is a simplified version --- the actual system has more properties and edge cases --- but the principle is clear. Every object is defined by a small set of boolean properties. The object itself does not contain interaction code. It has properties. The interaction code lives in the systems.

Layer 2: Universal Rules

The chemistry engine has a small set of universal rules:

1. Fire ignites flammable materials. A burning torch near grass ignites the grass. A fire arrow hitting a wooden crate ignites the crate. Any fire source near any flammable material produces ignition. No special cases.

2. Fire creates updrafts. Burning materials produce rising hot air. The paraglider can ride updrafts. This means any fire the player creates is also a potential launch pad.

3. Metal conducts electricity. During thunderstorms, metal objects attract lightning. Metal weapons, metal shields, metal armor --- all become lightning rods. The player can exploit this by throwing metal weapons near enemies or by switching to non-metal equipment during storms.

4. Metal is magnetic. The Magnesis rune can grab and move any metal object. Metal crates, metal doors, metal weapons dropped by enemies --- all are movable. This turns every metal object in the game into a potential tool: a bridge, a weapon, a shield, a projectile.

5. Water conducts electricity. Drop an electric weapon in water, and everything in the water takes damage. Stand in water during a thunderstorm, and you risk being struck. Water is both hazard and tool depending on context.

6. Wind affects fire and light objects. The Korok leaf creates a gust of wind. Wind pushes fire in the gust direction. Wind pushes rafts on water. Wind pushes objects off ledges. Fire and wind combine to create directional fire spread.

7. Cold freezes water and extinguishes fire. Ice arrows freeze water surfaces, creating walkable platforms. Cold environments freeze the player without warm clothing or food. Fire in cold environments melts ice and warms the area.

These rules are simple individually. A child can understand each one. But because they apply universally and because the world is dense with objects that have material properties, the interaction space is enormous.

Multiplicative Design in Action

The genius of the chemistry engine is what the GDC talk called multiplicative design: a small number of elements combining to produce a large number of interactions.

Consider what happens when the player encounters a Bokoblin camp in the rain:

• The wooden weapons the Bokoblins carry cannot be set on fire (they are wet).
• The player's fire arrows fizzle (rain extinguishes fire).
• Metal weapons attract lightning during the storm.
• The puddles on the ground conduct electricity.

The player realizes: drop a metal weapon in the puddle near the camp. Wait for lightning. The lightning strikes the metal weapon, conducts through the puddle, and electrocutes every enemy standing in the water.

Nobody designed "use lightning to electrify a puddle and kill Bokoblins." The designers created rain (which creates puddles), lightning (which strikes metal), electricity (which conducts through water), and damage (which hurts entities in electrified water). The solution emerged.

Now consider the dry version of the same camp:

• The grass around the camp is flammable.
• The wooden barrels store explosive materials.
• A torch is burning in the camp.

The player shoots a fire arrow into the grass. The fire spreads to the camp. The barrels explode. The explosion kills the Bokoblins and creates an updraft. The player paraglides over the destroyed camp and collects the loot from above.

Same camp. Different weather. Completely different emergent solution. The designer built one camp. The chemistry engine produced dozens of possible encounters.

💡 Intuition: Multiplicative design is the opposite of additive design. In additive design, each new feature adds one thing: a new quest, a new weapon, a new cutscene. In multiplicative design, each new feature multiplies everything else: a new material property interacts with every existing system, creating N new interactions where N is the number of existing systems. This is why Breath of the Wild feels inexhaustible despite having less authored content than Skyrim --- the interaction space grows multiplicatively, not linearly.

Teaching the Rules: The Great Plateau

The chemistry engine is only valuable if the player understands the rules well enough to exploit them creatively. Breath of the Wild solves this problem with the Great Plateau --- a contained tutorial area that introduces every major system interaction in a low-stakes environment.

On the Great Plateau, the player encounters:

• Fire + wood: A campfire near a wooden platform. The platform catches fire if the player carries a lit torch too close. The player learns: fire ignites wood.
• Fire + grass: Tall grass near an enemy camp. The grass burns when hit with a fire source. The player learns: fire ignites grass.
• Fire + updraft: A burning field produces rising air. The paraglider rides the updraft. The player learns: fire creates lift.
• Metal + Magnesis: Metal objects scattered near puzzles. The Magnesis rune grabs and moves them. The player learns: metal is interactive.
• Cold + ice: The cold mountain region requires warm clothing or food. Ice blocks melt near fire. The player learns: temperature is a system.
• Physics + everything: Boulders roll downhill. Trees fall when cut. Objects splash when dropped in water. The player learns: the world has physics.

Each interaction is presented in isolation first, then in combinations. By the time the player leaves the Great Plateau (roughly 2-5 hours into the game), they have a working mental model of the chemistry engine. They understand materials, they understand interactions, and they are ready to exploit the systems creatively in the open world.

The Great Plateau teaches without tutorials. There are no pop-up text boxes explaining that "fire ignites flammable materials." The teaching is environmental: the player sees fire near wood, the wood burns, and they learn the rule through observation. This is emergent teaching for emergent systems --- the systems teach themselves through their own behavior.

The Player as Scientist

The chemistry engine transforms the player from a consumer of content into a scientist. The player's primary activity in Breath of the Wild is not completing quests or defeating enemies. It is experimenting.

"What happens if I throw this metal box into the water during a storm?"
"What happens if I set this grass on fire and glide over it?"
"What happens if I freeze this waterfall and climb the ice?"
"What happens if I attach a bomb to a balloon and float it over the enemy camp?"

Each question is a hypothesis. Each attempt is an experiment. Each result teaches the player something about the rules. The player builds a mental model of the chemistry engine through experimentation, and then uses that model to solve problems in creative ways.

This is the feeling that emergence produces at its best: curiosity. Not the curiosity of "what is around the next corner?" (which is spatial curiosity, driven by content). But the curiosity of "what happens if...?" (which is systemic curiosity, driven by rules). Spatial curiosity is exhaustible --- eventually you have seen every corner. Systemic curiosity is inexhaustible --- as long as new combinations exist, new experiments are possible.

🎮 Design Spotlight: Tears of the Kingdom (2023) pushed the chemistry engine even further with Ultrahand and Fuse. Ultrahand lets the player attach any object to any other object, creating vehicles, weapons, machines, and structures from scavenged materials. Fuse lets the player attach materials to weapons, creating custom tools (a stick + a boulder = a hammer, a shield + a rocket = a rocket-powered escape). The combination space exploded: players built flying machines, walking robots, self-propelled boats, and elaborate Rube Goldberg contraptions. The sequel proved that the chemistry engine's depth was not exhausted by the first game --- it was barely scratched.

Lessons for Your Game

You are not building Breath of the Wild. You do not have a team of hundreds and a five-year development cycle. But the principles of the chemistry engine apply at any scale:

1. Define objects by material properties, not by specific behaviors. Your wooden crate should not have "burns when hit by fire arrow" code. It should have the property "flammable." The fire system handles the rest. This means every new flammable object you add automatically participates in the fire system without additional code.

2. Keep rules universal. Fire ignites all flammable things. Not "fire ignites wooden crates but not wooden bridges." Universal rules are learnable. Special-case rules are confusing.

3. Teach through environment, not through text. Place fire near wood early in the game. Let the player discover the interaction. Do not explain it. The discovery is the fun.

4. Create tools, not solutions. Give the player fire, wind, magnetism, and ice. Do not give them a "solve this puzzle" button. Let them combine the tools in ways you did not predict.

5. Trust the player. The chemistry engine works because Nintendo trusted players to be creative, to experiment, and to find solutions the designers never imagined. That trust is the most important design decision in the game. If you build the systems and provide the tools, the player will surprise you. Let them.

The chemistry engine is not a piece of technology. It is a design philosophy: build a world of materials, define consistent rules, and let the interactions generate the content. That philosophy works at any scale, in any engine, for any game.