Scope: Building Blocks & Construction = toy construction systems kids assemble piece-by-piece (interlocking plastic bricks, magnetic tiles/rods, wooden stacking blocks, and rod/strip engineering sets) — not marble runs, dollhouses, play dough, or simple one-piece toys (sibling leaves). This excludes marble-run/track-compatible expansion packs (e.g. Magna-Tiles marble-run add-ons) even when sold under a building-toy brand: those belong to the `marble-runs` leaf (catalog.json, published), not here.

## Seed
1. Equivalent of "weave": the connection mechanism (studs, notches, frictionless gravity-stacking, magnetic poles, nut-and-bolt) decides whether a build locks rigid, balances free, or snaps apart — not piece count or price, which is what listings lead with.
2. Pastry-chef persona notices first: the physical tolerance that makes pieces interchangeable at all — ABS injection-mold tolerance (clutch power), the unit block's 1:2:4 ratio, Meccano's fixed 1/2" hole pitch — the same way a ratio makes a recipe reproducible.

## Sources
[lego_wiki] https://en.wikipedia.org/wiki/Lego — ABS plastic, mold tolerance ~10-20 micrometers, Duplo is 2x standard brick dimensions for younger kids — fetched excerpt
[construction_set_wiki] https://en.wikipedia.org/wiki/Construction_set — connection-method taxonomy: studs, notches, no-connection/gravity stacking, struts-and-nodes, panels-and-screws, magnetic — fetched excerpt
[knex_wiki] https://en.wikipedia.org/wiki/K%27Nex — rod-and-connector system, plastic rods/connectors/gears/wheels, ages 5-12, Kid K'Nex for under-5 — fetched excerpt
[meccano_wiki] https://en.wikipedia.org/wiki/Meccano — metal strips/plates/girders joined by nuts and bolts, 1/2in (12.7mm) hole pitch, 5/32in BSW threads, only tools needed are screwdriver and spanner — fetched excerpt
[unitblock_wiki] https://en.wikipedia.org/wiki/Unit_block — unit block is 5.5x2.75x1.375in, 1:2:4 ratio; wood hierarchy maple > birch/beech > rubberwood — fetched excerpt
[toysafety_wiki] https://en.wikipedia.org/wiki/Toy_safety — choking-hazard labeling for under-3s, Magnetix recall after loose magnets caused injury/fatality — fetched excerpt
[magnatiles_faq] https://www.magnatiles.com/pages/faq — MABS plastic shell, embedded ceramic (ferrite) magnets, 3-inch base piece, cross-set compatibility — fetched excerpt
[ghk_buying_guide] https://www.goodhousekeeping.com/childrens-products/toy-reviews/g28568775/best-building-toys-for-kids/ — age ranges by brand/line (K'Nex 3+/5+, Magna-Tiles 4+, Lincoln Logs 3+), open-ended vs instruction-led play — fetched excerpt

## Branches
construction_type=stud_bricks -> brick_material, brick_scale, element_variety apply; magnetic/wood/engineering facets irrelevant (lego_wiki, construction_set_wiki).
construction_type=magnetic -> magnet_type, shape_system, casing_material apply; others irrelevant (magnatiles_faq, construction_set_wiki).
construction_type=wood_stacking -> wood_species, connection_mechanism, surface_finish apply; others irrelevant (unitblock_wiki, construction_set_wiki).
construction_type=engineering_sets -> connector_material, fastening_tool_requirement, motion_features apply; others irrelevant (knex_wiki, meccano_wiki).
age_band=toddler_18mo_3y -> pushes brick_scale toward large/toddler-safe and rules out small-fastener engineering sets (toysafety_wiki, lego_wiki Duplo fact). Note: the makes_irrelevant leg [fastening_tool_requirement, motion_features] is editorial reasoning, not a sourced claim — the cited sources support toddler-safe brick sizing, not that fastener-tool/motor features specifically become irrelevant at this age.

## Noise
Color/palette count (magnatiles_faq) — cosmetic, not a build-outcome differentiator.
BPA-free/phthalate-free/latex-free claims (magnatiles_faq) — baseline children's-plastic compliance, not a differentiator between sets.
Country of manufacture (lego_wiki: Denmark/Hungary/Mexico/China/Vietnam/US) — supply-chain trivia, not a buying factor.
Licensed theme/product line (knex_wiki licensed lines; lego_wiki minifigure themes) — marketing tie-in, covered loosely by element_variety, not its own facet.
Magnet temperature-exposure warning (magnatiles_faq) — real but niche care instruction, not a buying decision.

## Corrections (2026-10-26)
Added judgement naming CPSIA/ASTM F963 (US) and EN 71 (EU) as the governing standard behind choking_hazard_class/magnet_type, sourced to toysafety_wiki and magnatiles_faq (re-fetched via Wayback Machine, confirms "fully compliant with... CPSIA").
Relabeled piece_count_band's backfill_note to state the 100/500/1500 cutoffs are an editorial convention, not sourced to ghk_buying_guide (re-fetched; no band thresholds found in text).
Added a new facet `water_exposure_tolerance` (values dry_use_only/bath_and_splash_tolerant/dishwasher_safe), sourced to magnatiles_faq ("never be submerged") and ghk_buying_guide (Clixo "dishwasher-safe"), with vocabulary untouched since values are plain English.
Added a scope-boundary sentence excluding marble-run-compatible expansion packs from the magnetic/engineering branches, pointing to the published `marble-runs` leaf in catalog.json.
Relabeled wood_species from source:listing to source:researched and raised backfill_cost from medium to high, since its own backfill_note concedes species is often unstated on unbranded listings.
Flagged the age_band=toddler_18mo_3y -> makes_irrelevant [fastening_tool_requirement, motion_features] branch leg in notes.md as editorial reasoning rather than a sourced claim.
Added vocabulary entries for "Stud pitch" (compatibility_scope), "Polycarbonate" (casing_material), and "MDF" (wood_species) — previously undefined terms used only as facet values/definitions.