Domain: Natural Science — Physics
Target concept: Steel ball falling in water vs. falling in a glycerin column

Video generation prompt: Two parallel transparent vertical cylinders, one filled with water having a blue tint, the other with amber-colored glycerin; identical steel balls are released simultaneously and fall through the liquids.

Evaluation guide:
(i) Target concept: Viscosity-driven differential terminal velocity of identical dense spheres in two Newtonian fluids.
Minimum mechanism: Both fluids must be quiescent, homogeneous, and fully enveloping the spheres; spheres must be rigid, non-porous, and denser than both fluids; gravitational acceleration must be the sole driving force; no external flow, vibration, or surface tension dominance.
(ii)Storyline: Frame 1: Two upright cylinders side-by-side, filled, balls poised at top surface. Frame 2–3: Balls begin downward motion simultaneously (no delay, no stagger). Frames 4–end: Continuous descent with widening vertical separation; faster ball always ahead; no contact with walls/bottom; liquid surfaces remain undisturbed; viewpoint fixed, front-facing, orthogonal to cylinder pair; no occlusion, no cut, no zoom; entire experiment captured in single unbroken take from release to near-bottom passage.

Manual:
Experimental Procedure
1. Place two identical transparent vertical cylinders side-by-side in fixed frame.
2. Fill left cylinder with water dyed uniform blue; right cylinder with glycerin dyed uniform amber.
3. Simultaneously release one identical steel ball into each cylinder from same height at top.
Observation of Experimental Phenomena
The steel ball in the blue-tinted water falls rapidly and continuously. The steel ball in the amber-colored glycerin falls slowly and steadily. The speed difference is visually persistent and increases with time.
Summary of Experimental Principles
Glycerin has higher viscosity than water, resulting in greater drag force on the falling steel ball — causing slower terminal velocity. This demonstrates how fluid viscosity governs motion resistance for identical objects in different Newtonian fluids.

Prompt Grounding rubric:
level_1: No two parallel transparent vertical cylinders are visible, or only one cylinder appears, or the cylinder(s) are not oriented vertically, or no steel balls appear at all, or the prompt's basic setup (a blue-tinted liquid in one cylinder, an amber-colored liquid in the other, both balls released at the same time) is essentially absent from the scene.
level_2: Two vertical transparent cylinders stand side by side and contain liquid, but at least one required element is missing or wrong: for example, there is no visible blue tint in one cylinder, no visible amber color in the other, only one ball is present instead of two, or one ball clearly starts falling well before the other with no indication the release was meant to be simultaneous.
level_3: Two vertical transparent cylinders stand side by side in a fixed camera view; one cylinder's liquid is clearly blue-tinted and the other's is clearly amber-colored; one steel ball sits in each cylinder, and both balls begin moving downward within the same continuous shot, with no visible cut, pause, or hand intervening during the motion.
level_4: Two vertical transparent cylinders stand side by side in a fixed camera view, one liquid clearly blue-tinted and the other clearly amber-colored, with one steel ball per cylinder and both beginning to fall within the same continuous shot; in addition, both balls stay fully visible for the entire descent (never hidden behind a cylinder wall or the liquid's surface line), both cylinders keep their vertical alignment and transparency without warping, and no stray hands, on-screen labels, or graphic overlays intrude into the shot.
level_5: All of the setup and visibility criteria above are met, and additionally the two steel balls appear visually identical to each other in size, shape, and metallic shine; the blue tint stays confined to the water cylinder without bleeding into the glycerin cylinder or the background; the amber color stays confined to the glycerin cylinder with no visible hue change from top to bottom; the two cylinders closely match each other in diameter, wall thickness, and shape; and the background stays even and uncluttered, with every object holding the same position relative to the others from the first frame to the last.

Scientific and Causal Correctness rubric:
level_1: The two balls appear to fall at the same visible speed, or the ball in the amber glycerin falls faster than the ball in the blue water, or no speed difference between the two balls is observable at all, or the liquids' colors are assigned backwards relative to the prompt (e.g., the blue-tinted cylinder is treated as the glycerin, or the amber one as the water).
level_2: The ball in the blue-tinted liquid does fall visibly faster than the ball in the amber-colored liquid, but something else about the scene undercuts the demonstration: for instance the liquids are mismatched to their colors, the two balls do not look like matching steel balls, one ball's motion includes bouncing, rotating, or floating instead of straightforward falling, or one ball's descent stops or reverses partway down.
level_3: The ball in the blue-tinted water falls continuously downward and visibly faster than the ball in the amber-colored glycerin; both balls descend without stopping, reversing, or drifting sideways; the faster descent is consistently paired with the blue (water) cylinder and the slower descent with the amber (glycerin) cylinder, matching the expected outcome that the more viscous liquid slows the ball more.
level_4: The ball in the blue-tinted water falls continuously and visibly faster than the ball in the amber-colored glycerin throughout the descent, with no reversal, interruption, or lateral drift; in addition, this speed difference holds steady all the way down (neither ball catches up to or equalizes with the other), neither ball shows a sudden lurch, stall, or unexplained speed change that would contradict steady viscous drag, and both balls stay fully submerged with no surface contact or inconsistent trailing bubbles.
level_5: The speed difference between the two balls holds steady throughout with no lurches or stalls, and in addition the gap between the two balls' positions keeps widening from release to the end of the clip: at no point after release does the amber-liquid ball catch up to or draw level with the blue-liquid ball; the faster (blue-liquid) ball keeps a clean, undistorted leading edge, and the slower (amber-liquid) ball shows no odd wobble, tilt, or asymmetry that would be inconsistent with smooth viscous resistance.

Spatiotemporal Consistency rubric:
level_1: One or both transparent cylinders visibly teleport, rotate, or change size/position partway through the video; either steel ball vanishes and reappears rather than continuously falling; the frame cuts to a different angle or splits into multiple views; a cylinder's base or the liquid column itself breaks apart in a way liquid cannot physically do; or the overall lighting abruptly jumps brighter or darker with no cause tied to the action.
level_2: No teleportation or vanishing occurs, but at least one major continuity break is visible: for example, a ball appears already inside a cylinder without having entered through the liquid surface, the amber glycerin or blue water column visibly changes tint or transparency partway through for no physical reason, one cylinder's wall appears to warp or flex, motion freezes on a duplicated frame for a noticeably long pause, the background behind the cylinders suddenly shifts sideways or zooms, or the shadow cast by a cylinder or ball flips direction between frames with no matching change in the light source.
level_3: Both cylinders keep a fixed vertical position and shape throughout, both balls remain visible inside their own liquid column without disappearing or swapping cylinders, and motion is continuous with no freezing or skipping; the blue tint of the water and the amber tint of the glycerin stay the same shade and intensity from first frame to last, the metallic sheen on each ball's surface does not suddenly change brightness or color, and the direction of the lighting and shadows stays consistent with a single light source throughout; a minor imperfection is tolerable (e.g., a brief, faint flicker in the highlight on one ball) as long as it does not look like a break in the physical scene.
level_4: Both cylinders keep a fixed vertical position and shape throughout, both balls stay visible in their own liquid column, motion is continuous, and the tints and metallic sheen stay consistent as described above; in addition, each cylinder wall visibly refracts the background behind it in a consistent way throughout the fall (rather than looking like flat, undistorted glass in some frames and distorted in others), the liquid surface at the top of each cylinder stays level and undisturbed with no sudden ripple or meniscus collapse, and each ball's descent traces a smooth path with no unnatural jerks, hitches, or pauses.
level_5: Everything above holds with no lapses across the entire clip: the refraction pattern on both cylinder walls stays uniform and coherent from frame to frame, the balls' motion looks physically smooth with no ghosting or motion blur that mismatches their apparent speed, neither liquid column shows any flicker in its tint, and the background beyond the two cylinders stays completely static with no drifting or wobble. A viewer watching start to end sees one continuous, physically coherent scene.