Domain: Natural Science — Earth Science
Target concept: Darcy's Law: In medium-grained fine sand (0.1–0.5 mm), the gravitational term dominates permeability. After saline water contacts the sand surface, it infiltrates rapidly, and the wetting front descends significantly faster.

Video generation prompt: Two transparent beakers containing equal height and amount of fine sand are used. Clear water is slowly added to the left beaker using a pipette, while saltwater is added to the right beaker at the same speed using a pipette. The infiltration rates of the two liquids in the fine sand are compared.

Evaluation guide:
(i) Target concept: Gravitationally dominated infiltration-rate contrast between saline and fresh water in medium-grained fine sand (0.1–0.5 mm).
Minimum mechanism: The sand is cohesionless and non-capillary-dominant at this grain size, so capillary/surface-tension effects on infiltration are minor compared to gravity. By Darcy's law, gravity-driven flux scales with the fluid's density and inversely with its viscosity (q ∝ ρg/μ). The saline solution used here is denser than plain water, which increases its gravity-driven flux term (ρg); raising salinity also increases viscosity somewhat, but in this demonstration the density increase is taken to be the dominant effect, so once both fronts have entered the sand under otherwise identical conditions (same grain size, same delivery rate, same initial dryness), the saltwater wetting front is expected to descend somewhat faster and reach greater depth than the clear-water front.
(ii) Storyline: Frame 0: Two empty beakers side-by-side, dry fine sand evenly distributed at identical height. → Liquid delivery begins: pipette tip approaches left beaker opening → clear water contacts sand surface → wetting front initiates descent → pipette retracts → pipette approaches right beaker → saltwater contacts sand surface → wetting front initiates descent → both fronts descend continuously → by final frame, right-front depth > left-front depth. Visibility constraint: Camera fixed frontal view, full beaker height in frame, no occlusion, sand surface and wetting front fully visible throughout.

Manual:
Experimental Procedure
1. Place two identical transparent beakers side-by-side.
2. Fill both with equal height and amount of dry fine sand (0.1–0.5 mm).
3. Using a pipette, slowly add clear water to left beaker.
4. Using same pipette, slowly add saltwater to right beaker at identical speed.
Observation of Experimental Phenomena
— Wetting front descends deeper in right beaker (saltwater) than in left beaker (clear water) by final frame.
— Both liquids remain colorless and transparent; no surface pooling in either beaker.
Summary of Experimental Principles
— In medium-grained fine sand, the gravitational term dominates permeability (Darcy's Law) and capillary/surface-tension effects are minor at this grain size; the saline solution's higher density increases the gravity-driven flux term (ρg) relative to plain water, and this density effect is taken to dominate over the accompanying rise in viscosity, so the saltwater wetting front advances measurably faster under otherwise identical delivery conditions.

Prompt Grounding rubric:
level_1: Fewer than two transparent beakers containing sand are shown together (e.g., only one beaker, or a beaker paired with a non-beaker container); or no fine sand is visible in either container; or no pipette or liquid-delivery action ever appears; or the two containers are only ever shown one at a time rather than together as a matched pair.
level_2: Two transparent beakers are present, but they are placed one behind the other, diagonally, or stacked rather than as a matched side-by-side pair; or sand is added to only one of the two beakers; or the pipette is shown near a beaker but never releases any liquid onto the sand; or the sand fill height is obviously mismatched between the two beakers (one clearly higher than the other).
level_3: Two matching transparent beakers sit side-by-side with visibly equal sand fill height; a pipette dispenses liquid onto the sand surface in the left beaker and then, separately, onto the sand surface in the right beaker; both liquids appear colorless, matching the 'clear water' and 'saltwater' description; every object and step named in the prompt (two beakers, sand, pipette, sequential delivery to each) is present, even if the pace of delivery differs noticeably between the two beakers.
level_4: Everything required for the 3-point level is present, and in addition the pipette delivers liquid to the left and right beakers at a visibly matched pace (similar drop frequency and similar total dispensing duration for each), the two beakers are the same size and shape and framed at the same scale by the camera, and no extra props (labels, stirring rods, extra containers) appear anywhere in the shot.
level_5: Everything required for the 4-point level is present, and in addition the pipette is held at the same height and angle above the sand for both deliveries, the sand surface in each beaker is flat and undisturbed immediately before its own delivery begins, and the two beakers are visually identical in shape, size, and lack of labeling, so a viewer comparing the left and right delivery sequences side-by-side sees matched technique in every visible respect, not just matched timing.

Scientific and Causal Correctness rubric:
level_1: No wetting front or darkened sand band ever appears in either beaker (liquid only pools on the surface or vanishes); or the two wetting fronts stay at identical depth throughout; or the left beaker's (clear-water) front ends up deeper than the right beaker's (saltwater) front; or the sand grains look visibly too coarse (gravel-sized) or too fine (silty, clumping) for the described 0.1–0.5 mm regime.
level_2: A wetting front develops in only one of the two beakers; or fronts develop in both but their depths stay equal at every frame with no separation ever appearing; or one liquid shows a visible color or tint, contradicting the colorless clear-water/saltwater description; or a depth gap between the two fronts appears only briefly and then closes back to equal depth.
level_3: A downward-moving wetting front (a darker, damp band of sand) develops in both beakers; by the final frame the right beaker's (saltwater) front sits clearly deeper than the left beaker's (clear-water) front; both liquids stay colorless and transparent throughout; the sand in both beakers looks uniformly fine-grained with no clumping, dust, or pebbles.
level_4: Everything required for the 3-point level holds, and in addition the depth gap between the two fronts is already visible before the final frame and never closes back to equal depth or reverses at any point during descent; neither front pools or backs up at the surface once delivery has started.
level_5: Everything required for the 4-point level holds, and in addition the right beaker's (saltwater) front is at or ahead of the left beaker's (clear-water) front at every frame where both fronts are visible, with a gap that steadily widens or at least never shrinks, and each front's descent is smooth and monotonic rather than pausing and then jumping, consistent with a gravity-dominated Darcy flow in which the denser saline solution produces a larger downward driving force than the clear water.

Spatiotemporal Consistency rubric:
level_1: One or both beakers change position, size, or orientation between frames with no visible cause, as if relocated between shots; or the sand's fill height changes abruptly between frames without any visible pouring motion causing it; or the pipette vanishes mid-delivery and reappears at a different location; or a wetting-front boundary jumps down by roughly a third of the sand column or more within a single frame-to-frame interval instead of advancing gradually.
level_2: One beaker slides sideways, tilts, or rotates slightly between frames even though nothing in the scene should be moving it; or the pipette tip drifts out of the camera frame while liquid is still visibly being dispensed; or a wetting-front boundary blinks in and out of view instead of continuously advancing; or the sand surface switches from dry to visibly damp within a single frame with no gradual darkening in between.
level_3: Both beakers stay in the same position, orientation, and size in every frame; the pipette remains fully in view for the entire time it is dispensing into either beaker; each wetting-front boundary moves downward a small, continuous amount from one frame to the next, never jumping or moving upward; the dry-to-damp color change on the sand surface spreads gradually rather than switching instantly; neither beaker's outline is ever clipped or cut off by the frame edge.
level_4: Everything required for the 3-point level holds, and in addition the side-by-side spacing between the two beakers and their scale relative to the camera stay fixed across every frame; the pipette's path as it moves from the left beaker to the right beaker is a smooth, continuous motion rather than an instantaneous jump; the two beakers never overlap or occlude one another; neither wetting front's downward motion freezes for several frames and then jumps ahead.
level_5: Everything required for the 4-point level holds, and in addition each beaker's cylindrical outline keeps a constant, undistorted shape in every frame with no stretching, squeezing, or warping of the glass walls or rim; the visible sand-grain texture on both surfaces stays consistent from frame to frame with no sudden speckle or pattern flicker; the pipette's barrel and tip keep a constant visible width and transparency throughout each delivery; and the wet/dry boundary line in each beaker is always a single unbroken line rather than fragmenting into multiple disconnected patches.