Some of the hardest Primary 6 Science questions ask what happens when one part of a system stops working. A root cannot absorb water. A circuit branch opens. One prey disappears. A spring no longer returns to its original length. A leaf is blocked from light. The learner must reason from the changed condition through the system rather than recall a memorised sentence.
This guide develops what-if changes, system failure, counterfactual reasoning and failure-path analysis for PSLE Science. It is deliberately different from ordinary prediction practice: the focus here is diagnosing what breaks, what still works and how effects propagate through connected systems.
Return to the Primary 6 Science Learning Hub.
The failure rule
IDENTIFY THE FAILED PART → STATE ITS NORMAL JOB → FIND WHAT DEPENDS ON IT → TRACE THE FIRST DOWNSTREAM EFFECT → CHECK FOR ALTERNATIVE PATHWAYS → STOP AT THE REQUIRED ENDPOINT.
This is an eduKate reasoning routine, not an official SEAB formula.
Part I — Start with the normal function
Before reasoning about failure, ask what the part normally does.
Examples:
- roots absorb water;
- plant transport moves water to leaves;
- closed circuit paths allow electrical operation;
- food-web links show food sources;
- spring elasticity allows return toward original shape;
- respiratory system supports gas exchange;
- circulatory system transports materials.
If the normal job is unclear, failure reasoning becomes guesswork.
Part II — The first downstream effect is usually safest
If root water uptake falls, the first consequence is less water entering the plant.
Do not jump immediately to “the plant dies”.
A more disciplined chain is:
less water absorbed → less water transported to leaves → photosynthesis may be reduced → less sugar produced → growth may later be affected.
Part III — Failure does not always stop the whole system
Some systems have alternative routes.
A predator with two prey sources may continue feeding if one prey falls.
A branched circuit may keep one lamp operating when another branch opens.
Ask whether the failed component lies on every possible pathway.
Part IV — Essential part versus optional branch
An essential part sits on the only route required for the target outcome.
An optional branch provides one of several routes.
Example:
- single-series bulb path opens → entire path breaks;
- one parallel branch opens → another branch can remain complete.
Part V — Counterfactual reasoning means changing one fact
A counterfactual asks: What would happen if this condition were different while everything else remained as stated?
Good reasoning preserves the original world except for the specified change.
Do not silently change extra variables.
Part VI — Reverse counterfactuals
Sometimes the question asks what must have changed to produce an observed failure.
Example: a bulb that previously lit no longer lights.
Possible causes include:
- open circuit;
- failed cell;
- broken connection;
- failed bulb.
The diagram or evidence should eliminate alternatives.
Part VII — Failure cascades
A failure cascade occurs when one disrupted part affects several downstream components.
Example:
drought → fewer plants → less food for herbivores → herbivore decline → less prey for predators.
Each step needs a valid dependency. Do not skip links.
Part VIII — Delayed versus immediate failure
Some effects are immediate:
- open circuit → lamp goes off;
- switch opens → motor stops.
Others are delayed:
- reduced photosynthesis → growth changes later;
- prey decline → predator population changes over time;
- reduced water uptake → wilting or reduced growth later.
Part IX — Partial failure
A system may weaken rather than stop.
Examples:
- less light → reduced photosynthesis, not necessarily zero photosynthesis;
- one prey falls → predator may still feed on another;
- one circuit branch opens → other branches may work;
- less root uptake → reduced water supply, not instantly zero water everywhere.
Part X — Complete failure
Complete failure can occur when an essential dependency reaches zero.
Examples:
- no complete circuit path for a device;
- no light reaches a photosynthesising region;
- essential transport pathway is fully blocked in the simplified model.
Use complete-failure language only when the condition truly removes the required dependency.
Part XI — Original case study: plant transport blockage
A stem pathway carrying water upward is severely blocked.
Normal job: transport water to leaves.
First effect: less water reaches leaves.
Next: photosynthesis may be reduced because water is required.
Later: sugar production and growth may fall.
Original case study: one circuit branch opens
Two lamps are on separate branches. Lamp A’s branch opens.
First effect: current cannot travel through A’s branch.
Boundary: Lamp B can remain lit if its branch and shared supply path remain complete.
Original case study: predator loses one prey
Hawk H eats rabbits and mice. Rabbit numbers fall sharply.
Counterfactual: What if mice remain abundant?
Reasoning: H still has an alternative food source, so immediate hawk decline is not certain.
Original case study: spring does not return
A spring is heavily stretched and remains longer after the load is removed.
Normal model: spring returns toward original length after a moderate load is removed.
Failure clue: permanent deformation means the earlier elastic model no longer describes the spring fully.
Part XII — Diagnose where the chain breaks
For a failed system, draw:
A → B → C → D
Then mark the break:
A → B ✕ C → D
Ask which outcomes after the break are no longer supported.
Part XIII — Upstream versus downstream reasoning
Upstream asks what caused the failure.
Downstream asks what the failure affects.
Example: bulb does not light.
- Upstream: circuit open? cell flat? bulb failed?
- Downstream: no light output.
Keep the direction of reasoning clear.
Part XIV — Failure and evidence
Do not diagnose a failure from one symptom if several causes are possible.
Example: plant wilts. Possible causes include water shortage, root damage or transport blockage.
Additional evidence is needed to isolate the cause.
Part XV — Failure and fair testing
If intentionally removing a component to test its role, keep other relevant conditions comparable.
Example: testing light requirement should not also change water supply.
Counterfactual reasoning becomes stronger when only one factor changes.
Part XVI — Failure and energy systems
If an energy-conversion device fails, distinguish:
- energy source absent;
- transfer pathway broken;
- converter failed;
- output blocked.
These are different failure locations.
Part XVII — Failure and living systems
Living systems often compensate partly.
One leaf is shaded but other leaves still receive light.
One prey declines but another remains.
One root is damaged but others may still absorb water.
Do not treat organisms as single-path machines unless the question’s model does.
Part XVIII — The BREAK test
- B — Broken part: what changed or failed?
- R — Role: what does it normally do?
- E — Essential or alternative: is there another route?
- A — After-effect: what happens first downstream?
- K — Keep boundaries: where should the explanation stop?
This is an eduKate teaching mnemonic.
Part XIX — MCQ failure traps
- Assuming one branch failure stops every branch.
- Jumping directly to death or system collapse.
- Ignoring alternative prey or pathways.
- Changing extra conditions not stated.
- Confusing cause of failure with consequence of failure.
- Using a downstream symptom as proof of one specific upstream cause.
Part XX — Open-ended counterfactual frame
“If [part/condition] is removed or reduced, then [normal job] is reduced/stops. Therefore [first dependent stage] receives less/no [input], causing [requested downstream effect]. If an alternative pathway remains, the effect may be reduced.”
Where to connect
- Sequence, Cycles, Dependencies & System Pathways
- Boundary Cases, Exceptions & Edge Conditions
- Primary 5 What-If Changes & System Failure Reasoning
Retrieval checklist
- I start with the failed part’s normal job.
- I trace the first downstream effect before later effects.
- I check whether an alternative pathway remains.
- I distinguish partial from complete failure.
- I preserve all unchanged conditions in a counterfactual.
- I distinguish upstream cause from downstream consequence.
- I can map where a chain breaks.
- I recognise delayed versus immediate effects.
- I avoid diagnosing one cause from an ambiguous symptom.
- I can write a bounded failure explanation without jumping to collapse.
The quiet return
System-failure questions reveal whether a pupil understands function rather than labels. When a part disappears, the learner must know what job it performed, what depended on it and whether another route can take over.
Find the break. Name the normal job. Trace the first dependency. Check alternatives. Follow only as far as the question requires.
Return to the Primary 6 Science Learning Hub.
