Domain: Natural Science — Physics
Target concept: Release a hydrogen balloon in your hand, and verify that the balloon rises vertically without horizontal displacement, in contrast to an air balloon.

Video generation prompt: A person first releases a balloon filled with hydrogen, and then releases a balloon filled with air.

Evaluation guide:
(i) Target concept: Buoyancy-driven vertical motion difference between low-density gas (H₂) and ambient-density gas (air) in Earth"s atmosphere.
Minimum mechanism: Balloons must be sealed, same volume and material, released in same gravitational field and atmospheric conditions; hydrogen must remain contained and not mix with air; no external forces (e.g., wind, drafts, strings pulling) may act on balloons post-release.
(ii)Storyline: Frame 0–2: Person holds both balloons motionless at same height, side-by-side, fully visible. Frame 3–4: Hydrogen balloon is released: immediate onset of smooth vertical ascent with zero lateral drift. Frame 5–7: Air balloon remains stationary at release height while hydrogen continues ascending. Frame 8–10: Hydrogen balloon maintains upward trajectory; air balloon shows no vertical displacement, confirmed by fixed background reference (e.g., floor line, wall mark). Perspective: Static frontal view, centered on release plane; full balloons always in frame; no obstructions; lighting uniform and frontal to avoid shadow ambiguity.

Manual:
Experimental Procedure
1. A person holds two identical-looking balloons—one filled with hydrogen, one with air—in both hands at same height.
2. The person releases the hydrogen balloon first.
3. After a clear pause, the person releases the air balloon.
Observation of Experimental Phenomena
The hydrogen balloon rises vertically and steadily; the air balloon remains at or near its release height with no upward motion.
Summary of Experimental Principles
Hydrogen gas is less dense than air, producing net upward buoyant force; air-filled balloon has density ≈ ambient air, resulting in negligible net buoyant force and no sustained ascent.

Prompt Grounding rubric:
level_1: No balloon appears in the video, or only one balloon type is shown (hydrogen-filled or air-filled, not both), or no person is visible holding or releasing a balloon, or no release action ever takes place.
level_2: A person and two balloons appear, but the two balloons look identical (same color, size, and no labeling) so it is impossible to tell which is meant to be the hydrogen balloon and which the air balloon, or both balloons are released at the same instant instead of one after the other, or neither balloon is shown being held in the person's hand before release.
level_3: The person visibly holds two distinguishable balloons (e.g., differing color or a visible label) at roughly the same starting height; the hydrogen balloon is released first, and after a clear pause the air balloon is released; the person stays roughly in place through both releases.
level_4: Everything required for score 3 holds, and in addition the two balloons carry a clear visual cue identifying which is hydrogen and which is air (color-coding or a label such as "H2"/"Air"), both are released from a comparable hand height and position, and neither balloon is blocked from view by the person's arm, body, or any prop in the moments right after its own release.
level_5: Everything required for score 4 holds, and in addition both balloons stay fully visible on screen for the whole video (no cropping out of frame, no cut hiding either one), the release of each balloon is clearly shown (the hand visibly opening or letting go), and there is an unambiguous time gap between the hydrogen release and the air release.

Scientific and Causal Correctness rubric:
level_1: The two balloons behave the same way (both rise, both stay put, or both fall), or the air balloon rises while the hydrogen balloon does not, or there is no discernible difference in vertical behavior between them.
level_2: The two balloons behave differently, but the direction contradicts buoyancy (for example the air balloon visibly floats upward while the hydrogen balloon does not); or either balloon's motion is too erratic (tumbling, spinning, wandering sideways) to tell whether it is actually rising, sinking, or staying level.
level_3: The hydrogen balloon visibly climbs away from its release point on an upward path, while the air balloon shows no net gain in height (it stays near its release height or drifts slightly down); the vertical contrast between the two balloons is unambiguous to the eye.
level_4: Everything required for score 3 holds, and in addition the hydrogen balloon's climb continues without stalling, reversing, or sinking back down, the air balloon never shows a sustained upward drift at any later point in the video, and neither balloon pops, deflates, or visibly changes shape in a way that would explain its motion by something other than buoyancy.
level_5: Everything required for score 4 holds, and in addition the hydrogen balloon's rise is confirmed against a fixed background reference (such as a doorframe, wall marking, or floor line), showing it well above its release point by the end of the video, the air balloon's height relative to that same reference stays essentially unchanged from its release point, and the height gap between the two balloons visibly grows over time.

Spatiotemporal Consistency rubric:
level_1: A balloon disappears from view and reappears elsewhere in the frame, or its position, size, or orientation jumps abruptly between frames, or the screen flashes or briefly goes black, or a balloon's surface breaks apart (splitting or tearing) with nothing shown causing it.
level_2: A balloon's motion looks jerky or skips rather than moving smoothly, or a balloon spins or tumbles rapidly with no visible cause, or a balloon's shadow detaches from it or points a different way than the rest of the scene's lighting would suggest.
level_3: Each balloon's path through the frame is smooth and unbroken: no popping in or out of view, no sudden resizing, no unnatural bending or twisting; any visible shadow stays attached to its balloon and points a consistent direction; the room and background do not warp or shift.
level_4: Everything required for score 3 holds, and in addition the person's hand stays in contact with each balloon until the instant it is actually released (no balloon floating free before its release), any visible string or tether looks appropriately taut or slack for the balloon's current motion, and neither balloon disappears behind the person or a prop in one frame only to appear fully exposed the next with no visible motion connecting the two.
level_5: Everything required for score 4 holds, and in addition the two balloons' left-right placement in the frame never swaps or jumps unexpectedly, whichever balloon is moving changes position gradually from frame to frame rather than freezing and then jumping, and the camera itself never cuts, zooms, or shifts perspective during the sequence.