Domain: Natural Science — Physics
Target concept: Let two identical balls slide toward each other on a table and collide, verifying that they immediately rebound in opposite directions without coming to rest, which is a distinctive visual characteristic.

Video generation prompt: Place two identical and smooth balls on a horizontal table, facing each other. Release the two balls simultaneously so that they slide toward each other along a straight line with nearly equal speeds and collide at the center.

Evaluation guide:
(i) Target concept: Elastic head-on collision of identical masses in one dimension.
Minimum mechanism: Balls must behave as rigid, non-rotating, frictionless point masses with perfect energy and momentum conservation; table must provide no horizontal force (i.e., no friction or tilt); collision must be instantaneous and central.
(ii)Storyline: [Phase 1: Setup] Two identical balls stationary, aligned face-to-face on horizontal table, equidistant from center. [Phase 2: Initiation] Simultaneous release → forward sliding motion begins along straight, converging paths. [Phase 3: Collision] Contact occurs at a single point; no interpenetration or delay. [Phase 4: Rebound] Instant directional reversal for both balls along original line of motion. [Phase 5: Separation] Continued straight-line motion away from collision point, maintaining symmetry. Perspective must be fixed frontal/slightly elevated to unambiguously resolve directionality, alignment, and collision location; no occlusion permitted at any phase.

Manual:
Experimental Procedure
1. Place two identical, smooth, spherical balls side-by-side on a horizontal table, aligned so they face each other directly.
2. Release both balls simultaneously, allowing them to slide straight toward each other along the same line.
3. Observe their collision near the center of the path.
Observation of Experimental Phenomena
Both balls make contact and immediately rebound in opposite directions — no pause, no sticking, no passing through, no rest state — each reversing its direction of motion precisely.
Summary of Experimental Principles
Elastic collision between identical masses on a frictionless horizontal surface conserves both momentum and kinetic energy, resulting in complete velocity exchange: each ball rebounds with the speed and direction formerly held by the other.

Prompt Grounding rubric:
level_1: The clip never shows two balls together; or the two balls clearly differ in appearance (different color, size, or surface finish); or they are not resting on a horizontal table; or neither ball moves toward the other at all; or no collision between them is ever depicted.
level_2: Two similar balls sit on a horizontal table, but the setup breaks down somewhere: they are not aligned facing each other before release, one ball starts moving before the other (release is not simultaneous), their paths curve or angle rather than heading straight toward each other, or they meet far from the midpoint between their starting positions.
level_3: Two identical-looking balls (matching color, size, and surface finish) start at rest on a horizontal table, positioned facing each other; both are released at the same moment and travel in straight lines toward each other; they make contact near the midpoint between their starting positions; and each ball moves away from that contact point afterward, with no sudden jump in either ball's position at any point.
level_4: Beyond score 3, both balls stay fully visible for the entire sequence without passing behind anything or leaving the frame; each ball keeps the same color, surface sheen, and round shape before, during, and after contact; and neither ball visibly tumbles or spins in a way that changes its outline as it travels.
level_5: Beyond score 4, the two starting positions look equidistant from the eventual contact point, the two balls' approach paths look like mirror images of each other in speed and angle, contact happens at one clearly identifiable point with no visible overlap between the balls, and the entire sequence plays out in one unbroken shot with no cut, zoom, or camera-angle change.

Scientific and Causal Correctness rubric:
level_1: The two balls do not visibly change direction at contact: for example, they stop completely, appear to pass through one another, stick together and move as one, continue past each other in their original directions, or only one ball moves afterward while the overall motion looks like it violates momentum conservation (e.g., both balls appear to speed up after meeting).
level_2: Contact happens and at least one ball changes direction, but the response is incomplete or mismatched: one ball reverses while the other keeps moving forward, there is a visible pause before the reversal, both balls end up moving in the same direction afterward, or one ball's outgoing direction is not opposite to its incoming direction.
level_3: At the moment of contact, both balls reverse direction with no visible pause or rest in between, and each ball's outgoing direction is exactly opposite to its incoming direction; the reversal reads as immediate and continuous, with neither ball coming to a stop first.
level_4: Beyond score 3, the two balls also leave the collision at speeds that look similar to each other (neither one shoots off much faster or slower than the other), and after contact each ball's path stays straight along its original line of approach, without curving or veering off-axis the way it would if the ball were still carrying spin from before the collision.
level_5: Beyond score 4, the incoming paths of the two balls are exactly in line with each other and opposite in direction, and the outgoing paths are exactly in line and opposite as well, so the whole event reads as a clean mirror-image reversal; neither ball shows any sign of losing energy afterward (shrinking travel distance, stuttering motion, or slowing unnaturally); the rebound looks like a full, lossless swap of velocities.

Spatiotemporal Consistency rubric:
level_1: The balls jump from one position to another between frames with no continuous path between them; the tabletop surface flickers, warps, or disappears; the room lighting flashes suddenly; a ball's shape warps unnaturally (stretching, flattening, or briefly changing form); or the camera itself cuts or jumps mid-sequence.
level_2: A ball's motion looks jerky or skips steps rather than gliding smoothly; the edge or texture of the table shifts inconsistently between frames; a ball's shadow changes position or darkness abruptly with no change in the light source; or a ball momentarily disappears behind nothing, with no object present to explain it.
level_3: Both balls keep the same size and a consistent footing on the tabletop throughout, with no floating above or sinking into the surface; their motion is smooth and continuous from start through contact to separation; each ball's shadow points in one consistent direction and keeps roughly the same darkness for the whole clip, matching a single unmoving light source; and the balls never overlap or merge into each other's shape.
level_4: Beyond score 3, each ball's path is visibly straight both before and after contact, with no curving or sideways drift; the tabletop stays flat and does not tilt, buckle, or shift relative to the camera; each ball stays visually resting on the table's surface rather than hovering above it; and the two balls move at a scale that stays consistent with fixed background landmarks, with neither ball appearing to grow or shrink relative to the table.
level_5: Beyond score 4, the pacing of the whole event feels even (the run-up, the moment of contact, and the rebound follow one another with no sudden speed-ups, slow-downs, or freezes), and the distances between the balls and between each ball and the table's edge stay geometrically consistent from frame to frame, with no stretching, snapping, or interpolation glitches visible anywhere in the clip.