Domain: Natural Science — Marine Science
Target concept: Saline density separation, where plastic particles float and sand sinks

Video generation prompt: A transparent glass jar containing seawater was supplemented with a mixture of beach sand and colored plastic fragments, gently stirred, and then left undisturbed.

Evaluation guide:
(i) Target concept: Density-driven stratification in saline medium
Minimum mechanism: Sand density > seawater density > plastic density; gravitational settling dominates; no adhesive/cohesive forces override buoyancy/sinking; fluid viscosity permits observable separation within observation timeframe.
(ii)Storyline: [0–2s] Jar filled with seawater (static liquid surface); [2–4s] Sand and colored plastic added simultaneously into water (visible entry, initial dispersion); [4–6s] Single gentle stirring motion (rotational/swirling, brief duration, no splashing); [6–10s] Immediate cessation → sand descends continuously to form compact bottom layer → plastic fragments rise/stabilize in upper column → final state shows sharp horizontal interface between two immiscible solid layers separated by clear seawater; entire sequence viewed from fixed frontal angle, full jar in frame, no occlusion.

Manual:
Experimental Procedure
1. Fill transparent glass jar with seawater.
2. Add beach sand and colored plastic fragments into the jar.
3. Gently stir mixture once.
4. Immediately stop stirring and leave undisturbed.
Observation of Experimental Phenomena
After stirring stops, sand rapidly settles to the bottom while colored plastic fragments remain suspended or float in the upper water column — two distinct, non-mixing layers form.
Summary of Experimental Principles
Density-driven settling: beach sand is denser than seawater and sinks, while the plastic fragments are less dense than seawater and float or remain suspended — the density ordering sand > seawater > plastic governs which material settles and which stays buoyant.

Prompt Grounding rubric:
level_1: No transparent glass jar is visible; or no seawater is present; or no beach sand is present; or no colored plastic fragments are present; or no stirring action is shown; or no undisturbed stillness phase follows the stirring.
level_2: The transparent glass jar is present, but at least one of seawater, beach sand, or colored plastic fragments is missing; or the stirring action is absent; or no stillness phase follows the stirring.
level_3: The glass jar, seawater, beach sand, and colored plastic fragments are all present; a stirring action occurs and is followed by a stillness phase, but at least one object has the wrong material look (e.g., the sand reads as gravel or powder, or the plastic reads as paper or metal) or the plastic fragments are not visibly colored.
level_4: All four elements are present with correct type and appearance: a clear-walled cylindrical jar, seawater as a continuous liquid, tan/brown granular beach sand, and rigid, distinctly colored (non-earth-toned) plastic fragments; a brief stirring action is shown and is clearly followed by a stillness phase; no core object or action is missing or misidentified.
level_5: All elements from the 4-point level are present, and additionally: the glass jar shows a visible liquid meniscus at the seawater surface; the sand grains and plastic fragments are each rendered as separate, discrete particles rather than a single blurred mass; the stirring is brief and gentle (no splashing or jar movement); and the transition from stirring to stillness is visually clear, with the water surface calm and free of visible swirl trails at the moment stirring ends.

Scientific and Causal Correctness rubric:
level_1: No density-driven separation is observed: sand and plastic fragments remain mixed throughout at rest, with no visible tendency for either material to settle or float.
level_2: Materials move opposite to their known densities: e.g., plastic fragments sink to the bottom while sand floats or stays suspended, or both materials settle together, or separation happens only because plastic clumps together (not because of buoyancy).
level_3: Sand and plastic fragments become spatially separated at rest, but the vertical ordering is inconsistent with density: e.g., plastic fragments are partly embedded in the sand bed, or sand grains remain suspended mid-column, or plastic is fully submerged without floating or reaching the sand layer.
level_4: At rest, plastic fragments settle entirely above the sand layer and float or remain suspended in the upper water column, while sand forms a continuous layer at the very bottom of the jar; no plastic fragment rests on or below the sand bed, and no sand grains are suspended above the sand bed.
level_5: Plastic fragments remain buoyant and spread across the upper water column (not clumped only at the surface), while sand forms a compact, level bed at the bottom with no fragments mixed in; the seawater between the sand bed and the floating plastic is clear, with no sand grains drifting upward or plastic fragments sinking down into the sand.

Spatiotemporal Consistency rubric:
level_1: Severe continuity break: the jar's position or the seawater level jumps between frames; sand or plastic fragments vanish or appear without motion; the lighting abruptly switches (e.g., a sudden change in room brightness) with no transition; the stirring motion itself is discontinuous (freezes, reverses, or skips); or the jar visibly deforms, fractures, or breaks apart with no cause.
level_2: Noticeable break in continuity: the jar stays in place but sand or plastic instantly relocates from one position to another without visible travel; the stirring motion has no visible swirl trail or acceleration/deceleration; or during the stillness phase the water still shows persistent swirling or drifting particles inconsistent with stirring having stopped.
level_3: Small continuity flaws are present: slight jitter in the jar's position, a brief flicker in lighting during the stillness phase, or minor motion blur during stirring that never obscures the sand or plastic; sand or plastic may wobble briefly before settling but reaches a stable rest position by the end.
level_4: Motion throughout is physically continuous: the stirring shows smooth swirling motion that starts and fades gradually; sand is seen descending and plastic seen rising/floating without teleporting; the jar keeps a fixed position and undistorted shape, and lighting stays even with no unexplained shifts.
level_5: The jar stays perfectly still in the frame throughout; the stirring motion is brief, gentle, and free of overshoot or rebound; sand settles in one continuous downward motion without fragmenting or bouncing, and plastic fragments rise or float without bobbing sideways; the water surface stays flat and calm once stirring ends, with the stir-to-settle-to-still sequence flowing without visible cuts.