How do I explore?

1. Tap a dashed + space to drop in the block that belongs there. Each one is labelled with what it is waiting for.
2. Or drag a block off the shelf and put it anywhere you like. Blocks you join up a lot are easier to read when you keep them close together.
3. Join two blocks by dragging from the circle on one block's edge to the other block. The line that appears says what state the matter is in, and the KEY along the bottom names them.
4. Watch for a red wall. It means matter has reached that block and cannot get any further, so it is stacking up. Join that block on to something, and the wall goes away.
5. Add or take away one block, and watch what the rest of the cycle does.

What should I do/notice?

• Plants are the way matter gets in. A plant takes matter from the air, from water and from the soil, and turns it into food. Nothing else on the board can do that. A plant needs all three — look at the three circles down its left side and see which ones nothing is joined to yet.
• Animals eat and are eaten. Matter comes in one side as food and leaves the other side as the animal's own body, for whatever eats it or breaks it down next. Rabbits and mice eat plants; foxes and owls eat animals. Look at the mark on a food circle to see which it takes.
• Decomposers are the way matter gets back. Fungi, worms and bacteria break down dead plants and dead animals, and return matter to the air, the water and the soil — ready for a plant to use again. Take them away and watch matter pile up with nowhere to go.
• Use the key at the bottom. Tap Air, and every line except the air lines fades back, so you can follow just that one all the way round. Tap it again, or click the board, to see them all.
• Where did the matter in the rabbit start out? Follow it back and see if it leaves the cycle anywhere.

What this model leaves out

A model is never the whole picture. Leaving things out on purpose is how a model makes one idea easier to see. Here is what we left out, and why.

• Sunlight. A plant needs light as well as air, water and soil. There is no sun on this board, because light is energy rather than matter, and this model follows matter.
• Breathing and drinking. Real animals and decomposers take in air and water all the time, and give off waste. Here, matter only gets into them through their food, and only leaves through their bodies.

Everything you can see on the board still works the way it really works. There is just less of it, so the cycle is easier to follow. What would you add back first?

Standards

• NGSS 5-LS2-1: Develop a model to describe the movement of matter among plants, animals, decomposers, and the environment.
• NGSS MS-LS2-3 (extends): Develop a model to describe the cycling of matter and flow of energy among living and nonliving parts of an ecosystem.

Design intent

This is a model-building sandbox. The practice is Developing and Using Models and the crosscutting concept is Systems and System Models; the artifact is the dynamic matter-cycle picture itself, so the sim deliberately does not export a dataset.

The model treats the environment (air, water, soil) as first-class components alongside the organisms, matching the evidence statement's five components. The conceptual heart is the clarification statement: matter that is not food (air, water, decomposed materials in soil) is changed by plants into matter that is food. So the matter a student watches move up the food chain has a visible origin — it enters through the plant.

The three kinds of not-food matter are named for their sources — air, water and soil — which is the clarification statement's own list rather than a set of substances we chose. Naming the first one "carbon" would have raised the question of oxygen, since air holds both, and answering it means gas exchange and a different standard. Calling it air keeps the model qualitative: matter in, food out, no molecular chemistry and no naming of gases, which is what the assessment boundary asks for.

Where the line is drawn

Only plants take matter from the environment. Only decomposers give it back. Every other block simply eats and is eaten. That single rule is what keeps a complete cycle to about eight connections instead of sixteen, and it is worth defending as the sim grows: every organism really does exchange matter with its surroundings, so there is always another true link that could be added. Those are the links that cross the whole board rather than joining a neighbour, and they are what makes a web unreadable.

A plant is an and-gate: it makes no food at all until air, water and soil are all reaching it. Each reservoir supplies exactly one of the three, so every reservoir on the shelf has a job, and it is what gives decomposers their purpose — if a plant did not need what the soil holds, matter returning to the soil would change nothing.

A decomposer is an or-gate: dead plant matter or dead animal matter will do. Its two mouths sit clearly apart, where an and-gate's inputs are pulled together until they touch. That spacing is the only thing distinguishing the two ideas on screen, and students read it without being told.

Decomposers return to all three reservoirs, not just the soil. If they returned only to soil, a student following the air would watch it leave the sky and never come back — air would be a one-way source, which is the opposite of "matter cycles between the air and soil."

Known simplifications

These are deliberate, and worth knowing before a student asks:

• Animals do not breathe or drink here. LS2.B says organisms obtain gases and water from the environment and release waste back into it, and that is true of every animal on the board. It is not modelled because giving each organism its own environment exchange doubled the number of connections and made the web unreadable. In this model, matter reaches an animal through its food and leaves through its body.
• Decomposers do not breathe either, for the same reason, though decomposition is respiration.
• Plants respire too, and this is not shown, so that "plants are the way matter gets in" stays unambiguous.
• There is no sunlight here. A plant needs light as well as air, water and soil, and this model shows it making food as soon as all three arrive. Light is energy rather than matter, and this sim tracks only matter, so the sun would be a source that never sends anything round the loop. Expect students to raise it, particularly alongside 5-PS3-1, where they model the energy in an animal's food as having come from the sun.
• Waste is not a separate output. Matter an animal does not keep leaves with its body to a decomposer, rather than by its own route.

None of these change where matter ends up — it still cycles, and it is still conserved. They change how many lines a student has to read to see it.

How this differs from Same Story, Different Cast

Two ecosystem sims, deliberately different jobs. This one builds one cycle: the student assembles a single working ecosystem and traces matter around it, which is a modelling practice. Same Story, Different Cast (MS-LS2-2) observes across sites: the student visits several ecosystems and finds the same kinds of relationship recurring in each. Neither is a food-web builder twice over — if they start to converge, one of them is in the wrong place.

Discussion prompts

• Where does the matter in the rabbit originally come from? Trace it back through the model.
• What happens to matter when you remove the decomposers? Where does it get stuck, and why can't it return to the plants?
• Compare the air path, the water path and the soil path using the key at the bottom. Where are they the same? Where are they different?
• Animals really do breathe and drink, and this model leaves that out. Where would those arrows go, and what would they change?
• Build two plant-eaters that share one plant. What happens to the plant, and what would happen if a new animal arrived with nothing to eat it?