Domain: Healthcare — Skin Physiology
Target concept: Erythema reaction (friction)

Video generation prompt: A clean forearm is displayed. A hand with a smooth pad rubs the same skin area back and forth with moderate force for several seconds, then stops, and the skin is left exposed for observation.

Evaluation guide:
(i) Target concept: Friction-induced erythema as a transient, localized, vascular physiological response.
Minimum mechanism: Mechanical shear stress on epidermis must trigger neurogenic and endothelial-mediated arteriolar dilation in the dermal plexus, resulting in increased perfusion and hemoglobin-dependent redness, requiring intact microvasculature, normal autonomic tone, and absence of vasoconstrictive agents.
(ii)Storyline: [Phase 1: Baseline] Clean forearm fully visible, static, evenly lit, no redness. [Phase 2: Intervention] Hand enters frame holding smooth pad; pad contacts skin; back-and-forth linear motion begins on fixed locus; motion is continuous, planar, and confined. [Phase 3: Cessation] Motion stops abruptly; hand either lifts cleanly or holds pad stationary without pressure; no repositioning. [Phase 4: Observation] Skin remains fully exposed, motionless, and uniformly lit for ≥2 seconds; erythema emerges *only* in rubbed zone, with homogeneous pink-to-red hue, sharp spatial boundaries matching pad contact area, and no fading during observation. Perspective: Fixed frontal close-up (elbow to wrist), no camera movement, no occlusion, consistent focal plane throughout.

Manual:
Experimental Procedure
1. Display a clean, bare forearm centered in frame.
2. A hand holding a smooth, matte pad contacts the forearm skin.
3. Rub the same skin area back and forth with moderate, steady pressure for several seconds.
4. Stop rubbing completely and withdraw hand slightly while keeping pad in place or lifting cleanly.
5. Leave skin fully exposed and motionless for observation.
Observation of Experimental Phenomena
A localized, homogeneous pink-to-red erythematous area appears exclusively within the rubbed region after rubbing ceases and persists during observation; no swelling, blistering, or discoloration outside this zone.
Summary of Experimental Principles
Erythema reaction is a transient, friction-induced vasodilation of superficial capillaries; it manifests as reversible, localized reddening due to increased blood flow and capillary engorgement — a normal physiological response to mechanical stimulation of healthy skin.

Prompt Grounding rubric:
level_1: No forearm is shown, or no hand/pad rubbing action occurs, or the skin is never left exposed for observation after rubbing: at least one of the three stages (clean forearm, rubbing, post-rub observation) is missing entirely.
level_2: The forearm shown has visible hair, jewelry, or clothing covering part of the skin; the hand is present but is not holding anything resembling a smooth pad (e.g., bare fingers only, or a clearly textured/rough object); the motion shown is tapping, pressing, or single-direction wiping rather than a repeated back-and-forth rub; the video ends before any post-rub still-exposure period.
level_3: A bare forearm is shown, a hand holding a smooth pad contacts the skin and rubs back and forth for several seconds over the same spot, the rubbing stops, and the skin is then left visible and still: all three stages occur in the correct order, though the pad's texture or the forearm's starting condition may not perfectly match the prompt (e.g., the pad looks slightly textured, or the forearm already had a mark before rubbing).
level_4: Everything in the 3-point level holds, and in addition the pad is smooth and matte with no obvious texture or pattern, the forearm is bare with no gloves, sleeves, or digital overlays, no extra hands or objects enter the frame, and the rubbing stays confined to one patch of skin rather than drifting across the arm.
level_5: Everything in the 4-point level holds, and in addition the pad's surface looks uniformly smooth and non-glossy throughout, the forearm stays fully in view and evenly lit from the first frame to the last, the back-and-forth rubbing covers the same patch of skin without drifting to a new spot, and the final observation period shows no repositioning of the hand, no refocusing, and no camera cut.

Scientific and Causal Correctness rubric:
level_1: No pink or red mark ever appears on the skin, or the skin's color never changes from its starting tone, or a color change appears with no connection to where the rubbing happened.
level_2: A red or pink mark appears somewhere other than the rubbed patch (e.g., on the palm, surrounding skin, or in the background); or the mark appears before rubbing starts; or the mark visibly fades away while rubbing is still happening; or the color that appears is not a plausible skin-reddening tone (e.g., purple, yellow, or blue).
level_3: A pink-to-red mark appears only on the patch of skin that was rubbed, and only after the rubbing stops; the mark stays visible for the rest of the observation period rather than fading; no swelling, blistering, or sweating appears anywhere on the arm, though the mark's edges or evenness may look somewhat rough or patchy.
level_4: Everything in the 3-point level holds, and in addition the reddened area lines up with the patch the pad actually touched, the color is fairly even pink-to-red across that patch, the mark becomes visible within roughly the first half of the observation period after rubbing stops rather than appearing much later, and no unrelated skin changes (e.g., paling or blotching elsewhere) compete with the erythema.
level_5: Everything in the 4-point level holds, and in addition the reddened patch's outer boundary roughly tracks the shape of the pad's contact area rather than spreading well beyond it, the color fills in gradually rather than snapping on as a hard-edged block, and nothing about the lighting, blur, or exposure gets in the way of seeing the redness clearly.

Spatiotemporal Consistency rubric:
level_1: The forearm or the hand vanishes and reappears partway through the video; the camera suddenly cuts from a wide shot to a close-up or back; the lighting suddenly flashes or a shadow sweeps across the scene; the skin surface visibly warps or stretches; the rubbing motion freezes on a frame and then jumps ahead.
level_2: The pad's position jumps to a new spot without any visible hand motion carrying it there; the forearm rotates or shifts without the hand moving along with it; a sleeve or object edge appears and disappears in the frame without explanation; the direction of the lighting or shadow changes noticeably between the rubbing and observation phases; the motion visibly stutters or repeats a frame.
level_3: The forearm stays in the same place in the frame throughout, the hand moves smoothly back and forth without jumping, the pad stays touching the skin for the whole rubbing phase, nothing in the scene disappears or reappears without cause, and the lighting stays the same brightness and direction from the rubbing phase into the observation phase.
level_4: Everything in the 3-point level holds, and in addition the forearm's outline and the pad's outline never stretch, skew, or warp; the pad-to-skin contact never visibly breaks during rubbing; the camera shows no sudden zoom or drift; the shadow cast by the hand keeps a consistent shape and direction; the rubbing motion runs at a steady pace with no visible speed-up or slow-down.
level_5: Everything in the 4-point level holds, and in addition the hand traces a clean, repeating back-and-forth path over the same patch of skin without wandering, the pad and the forearm skin behind it stay in consistent relative position with no visible depth mismatch, and during the still observation phase the hand and pad show no visible repositioning or drift from where the rubbing ended.