Domain: Natural Science — Cell & Developmental Biology
Target concept: Under the light microscope, onion epidermal cells stained with iodine show the nucleus.

Video generation prompt: Under the light microscope, onion epidermal cells on a slide were stained with iodine and brought into focus for examination.

Evaluation guide:
(i) Target concept: Optical visualization of plant cell nuclei via iodine-mediated contrast enhancement in a light microscope preparation.
Minimum mechanism: Iodine must be in physical contact with hydrated onion epidermis to bind intracellular components; sufficient time must elapse for diffusion and binding (implied by static post-staining view); microscope optics must resolve ~5–10 µm features to distinguish nucleus from cytoplasm; nucleus must reside within cell boundary and exhibit higher absorbance than surroundings due to iodine complexation.
(ii)Storyline: [Frame 1–2] Glass slide enters view, flat and empty. [Frame 3–4] Onion epidermis placed centrally: thin, semi-transparent, irregular margin. [Frame 5–6] Amber-brown iodine droplet contacts tissue and spreads continuously. [Frame 7–8] Coverslip lowered smoothly; meniscus forms and stabilizes. [Frame 9–10] View transitions to stable, in-focus microscope field: rectangular cells appear with crisp walls; within each, a single, darker, rounded nucleus is centered and uniformly contrasted. All nuclei co-appear, co-vanish, and maintain fixed spatial relations to their host cells and neighbors.

Manual:
Experimental Procedure
1. Place fresh onion epidermis flat on a clean glass slide.
2. Add one drop of iodine solution onto the tissue.
3. Gently lower a coverslip, avoiding bubbles.
4. Place slide on microscope stage and focus under low-to-medium power until cells are sharp.
Observation of Experimental Phenomena
Rectangular plant cells with distinct cell walls; each cell contains one centrally located, darker, rounded nucleus stained by iodine.
Summary of Experimental Principles
Iodine binds proteins and nucleic acids concentrated in the nucleus, increasing its optical density relative to the surrounding cytoplasm and making it visible as a darker region under white-light microscopy.

Prompt Grounding rubric:
level_1: No onion epidermal tissue, no glass slide, no iodine solution, and no in-focus microscope view appear; the scene has nothing to do with preparing or examining a stained slide.
level_2: At least one correct object appears, such as a slide or a droplet of liquid, but there is no recognizable onion epidermis, no iodine being applied, and no steady in-focus microscopic field; the actions do not follow a placement-then-staining-then-viewing order.
level_3: Onion epidermal tissue sits on a glass slide, an amber-brown iodine solution is visibly applied to it, and the sequence settles into a static, in-focus microscope view showing cellular structures; whether a nucleus-like feature is distinguishable inside the cells is not yet required.
level_4: Everything at the 3-point level is present, and additionally, the onion epidermis, the applied iodine, and the in-focus cellular view are all correctly arranged and connected in sequence, and a darker, rounded structure appears inside at least some of the visible cells, though its placement or contrast may be uneven across cells.
level_5: Everything at the 4-point level is present, and additionally, the epidermis lies flat and continuous on the slide, the iodine solution wets the tissue evenly with no pooling or dried patches, the microscope view stays sharply focused and stable, and multiple adjacent cells with intact walls each show one centrally placed, similarly shaped and similarly contrasted darker structure, appearing together rather than some cells showing it and others not.

Scientific and Causal Correctness rubric:
level_1: The nucleus-like feature appears in a color other than a natural iodine-brown tone, such as bright white, blue, green, or red; it appears outside cells or spanning across cell boundaries; the iodine is missing or replaced by a clear, water-like liquid; or the cells show no visible structure at all, with no walls and no rectangular shape.
level_2: Iodine is present but the wrong color, such as purple rather than amber-brown, or it sits on a surface other than the tissue; the dark nuclear structures look like irregular smudges or streaks rather than rounded bodies; the cells lack defined walls or look rounded and animal-like rather than rectangular plant cells; there is no visible connection between the iodine reaching a cell and that cell's nucleus darkening.
level_3: The iodine is amber-brown and touching the tissue; the cells show rectangular outlines with visible walls; darker, rounded structures appear inside cells, but some cells that are otherwise visible are missing this structure, or the contrast is reversed with the cytoplasm darker than the structure, or the structure sits at a cell's edge rather than near its center.
level_4: The iodine is amber-brown and evenly touching the tissue; the visible cells are rectangular with intact walls; each visible cell contains exactly one rounded, roughly central, darker-than-cytoplasm structure; however one or more of these structures looks internally inconsistent, such as speckled or hollow rather than evenly toned, or is clearly too large or too small relative to its cell in two or more cells.
level_5: The iodine is amber-brown and wets the tissue fully with no gaps or overruns; every visible cell is rectangular with continuous walls and contains exactly one nucleus-like structure that is rounded, centrally placed, evenly darker than the cytoplasm, uniform in tone throughout, and sized proportionally to its cell; this darkening appears only where the iodine has reached the tissue, with no such structure appearing in any unstained area.

Spatiotemporal Consistency rubric:
level_1: The scene jumps between a wide view and the microscope view with no transition; the slide appears already fully stained with no application step shown; the lighting suddenly flips from bright to fully dark mid-sequence; the cell walls flicker or vanish between frames; or a nucleus-like structure appears or disappears with no visible cause.
level_2: The microscope view shakes, drifts, or rotates unsteadily; the iodine appears on the tissue instantly with no visible spreading; the tissue moves independently of the slide beneath it; a nucleus-like structure jumps to a new position between frames with no visible motion; or a cell wall breaks or warps mid-view.
level_3: The view stays fixed on the same microscope field throughout; the slide and tissue stay still; the iodine spreads continuously outward from where it lands; the cell walls stay intact and rigid; once in focus, each nucleus-like structure stays steadily visible, with only a minor momentary flicker or a brief passing shadow covering part of one structure for an instant being acceptable.
level_4: Everything at the 3-point level is present, and additionally, there is no jitter, no unexplained occlusion, and no drift; the slide, tissue, iodine, cells, and nuclear structures all keep the same spatial relationship to one another; any motion, such as the droplet settling, looks smooth and physically plausible; the lighting stays even and stable throughout.
level_5: Everything at the 4-point level is present, and additionally, the slide, tissue, iodine, cells, and nuclear structures all keep the same relative positions and proportions across every frame; the iodine's spread is continuous and unbroken from first contact onward; any focus change is smooth and moves in one direction only; there is no flicker, no warping, and no duplicated or skipped frame; every structure stays fully visible and stable in shape from the first frame to the last.