Domain: Natural Science — Biochemistry & Molecular Biology
Target concept: Methylene blue staining of human buccal epithelial cells on a microscope slide

Video generation prompt: A swab is used to collect cells from the inside of the mouth, which are then spread on a slide, stained with methylene blue, covered with a coverslip, and observed under an optical microscope.

Evaluation guide:
(i) Target concept: Visualization of human buccal epithelial cell morphology via vital basic dye staining.
Minimum mechanism: Methylene blue must bind preferentially to nucleic acids in intact, unfixed cells; staining requires aqueous medium and sufficient dwell time for diffusion; coverslip mounting must preserve cell monolayer without crushing or washing away cells.
(ii)Storyline: [Frame 1–2] Swab contacts inner cheek mucosa (visible moisture transfer). → [Frame 3–4] Swab tip strokes slide surface, leaving translucent smear. → [Frame 5–6] Methylene blue droplet contacts smear, spreads radially with glossy meniscus. → [Frame 7–8] Coverslip approaches at ~45°, contacts slide edge, then lowers smoothly to exclude air. → [Frame 9–10] Microscope view reveals focused field: ≥3 well-separated cells, each with continuous membrane, light-blue cytoplasm, and centrally located darker nucleus; all cells lie in same focal plane with no stacking.

Manual:
Experimental Procedure
1. Swab gently rubbed inside cheek → cells collected.
2. Swab streaked across clean glass slide → thin cell layer formed.
3. One drop methylene blue applied → cells stained blue.
4. Coverslip lowered diagonally → air bubble excluded.
Observation of Experimental Phenomena
Under microscope: flat, irregular epithelial cells with distinct light-blue cytoplasm and darker-blue round nuclei, arranged in monolayer beneath transparent coverslip.
Summary of Experimental Principles
Methylene blue is a basic dye that binds to acidic components (e.g., DNA in nuclei), causing selective blue staining; human buccal cells are eukaryotic, non-keratinized squamous epithelium with visible nuclei when properly stained and mounted.

Prompt Grounding rubric:
level_1: No swab, no slide, no methylene blue bottle, no coverslip, no microscope visible; none of the four core steps (swabbing, spreading, staining, covering) appear; no human face or oral cavity region shown.
level_2: At least one correct object present (e.g., swab or slide), but zero correctly sequenced actions; e.g., coverslip appears before staining, or microscope appears without slide under view; swab is shown but never contacts mouth or slide.
level_3: All four core objects present (swab, clean slide, methylene blue container, coverslip) AND at least three of the four core actions occur in correct relative order: (1) swab contacts inner cheek, (2) swab transfers material onto slide surface, (3) methylene blue is applied to spread cells, (4) coverslip is placed over stained area; microscope may be shown but not required for PG.
level_4: All four core objects present AND all four core actions occur in correct relative order AND all actions are performed on the same slide (no slide swap or replacement); swab visibly contacts cheek then same swab touches slide; methylene blue is visibly dispensed onto the same area where cells were spread; coverslip is placed directly over that stained area.
level_5: All conditions for score 4 met AND spatial continuity preserved: swab remains in frame or clearly tracked between cheek → slide → staining step → coverslip placement; no cut-away or scene break between any two consecutive core actions; all actions occur within a single continuous shot or seamlessly edited sequence preserving causal linkage and object identity.

Scientific and Causal Correctness rubric:
level_1: Cells appear unstained (colorless/transparent), or stain is bright blue but applied *before* cell transfer, or cells appear as dense clumps with no individual morphology, or coverslip shows air bubbles *and* cells are fully obscured, or microscope view shows no cells at all or only debris.
level_2: Methylene blue is applied but cells remain colorless; OR stain appears purple/violet/red instead of blue; OR cells are stained but show no distinct nuclei (no darker central spot); OR coverslip is placed but no cell layer is visible beneath it; OR microscope view shows uniform blue smear with no cellular structure.
level_3: Cells are visibly stained blue (not purple/red/green), and individual epithelial cells are discernible as flat, irregularly shaped, thin structures with visible nuclei (darker blue central region); coverslip is placed and cells remain visible beneath it; no air bubbles fully occluding the cell layer.
level_4: All conditions for score 3 met AND nuclear staining is consistently darker than cytoplasm across ≥3 clearly resolved cells; cell borders are sharply defined (not blurred or fused); background is pale blue (not saturated dark blue); no evidence of over-staining (e.g., precipitated dye crystals) or under-staining (e.g., faint halo-only cells).
level_5: All conditions for score 4 met AND every visible epithelial cell displays: (a) intact plasma membrane (continuous blue outline), (b) centrally located, rounded, darker-stained nucleus, (c) homogeneous light-blue cytoplasm, (d) no overlapping layers — all cells lie in single monolayer plane beneath coverslip; microscope view shows crisp focus with no chromatic aberration or halos around nuclei.

Spatiotemporal Consistency rubric:
level_1: Objects teleport (e.g., swab disappears and reappears in new position/orientation without motion), or slide rotates >90° without hand contact, or lighting changes abruptly between frames, or coverslip snaps into place with no descent motion, or microscope eyepiece view jumps discontinuously.
level_2: At least one major spatiotemporal violation: e.g., swab moves toward cheek but stops short and no contact occurs; methylene blue droplet appears suspended mid-air above slide then vanishes; coverslip flickers in/out of transparency; microscope view shows identical frame repeated >3 times without change.
level_3: All object motions are physically plausible: swab contacts cheek with bending deformation, slide remains stationary during spreading/staining, methylene blue flows as viscous liquid upon contact, coverslip descends smoothly and settles with slight compression; no abrupt lighting shifts or shadow jumps; no object duplication or erasure.
level_4: All conditions for score 3 met AND temporal continuity preserved: no skipped intermediate states (e.g., methylene blue must visibly wet slide surface before spreading; coverslip must show initial edge contact before full coverage); no object occlusion inconsistencies (e.g., swab fully visible behind slide when logically in front).
level_5: All conditions for score 4 met AND perfect spatial anchoring: slide maintains fixed position and orientation throughout; swab tip remains centered on slide during spreading; methylene blue application occurs only within slide boundaries; coverslip aligns precisely with slide edges (no overhang/underhang visible); microscope view remains stable with no drift or jitter during observation phase.