You ask for pouring water into a glass and the water flows sideways. Or a flag waving in wind and the cloth ripples in patterns no real cloth would make. Or a ball rolling down a hill and it pauses, accelerates wrong, or rolls uphill briefly. The action is recognizable but physically impossible.
AI video models learn from videos, but they don’t have a physics engine. They’ve seen many examples of water pouring, but they don’t understand that water always flows downward — they’re statistically reconstructing what pouring “looks like.” For simple cases this works; for edge cases it fails dramatically.
Common causes
Ordered by hit rate, highest first.
1. Model has limited physics understanding
Current video models (Runway, Pika, Kling, Sora, Veo) all have weak physics. They struggle with:
- Fluid dynamics (water, smoke, fire)
- Cloth dynamics (flags, capes, hair)
- Multi-object collisions (balls, dominoes)
- Gravity-dependent motion (falling, jumping, dropping)
- Friction-dependent motion (sliding, rolling on surfaces)
How to spot it: your shot involves one of these phenomena. Expect issues.
2. Action is complex / multi-stage
pours water from a kettle into a cup until full involves: tilt, flow, accumulation, level rise, stop. Five physics moments to get right.
How to spot it: action has multiple physical stages.
3. Reference image doesn’t match the action
You uploaded a still of a kettle held over a cup, asked for “pouring.” Model has to interpolate the full pour from a single frame — too much extrapolation, too much room for error.
How to spot it: source frame doesn’t show the action in progress.
4. Tool / model version weak on physics
Sora and Veo currently handle physics better than Runway / Pika. Smaller / older models are worse.
How to spot it: same shot on Sora / Veo works, on tool X doesn’t.
5. Object is one the model hasn’t seen often
Unusual objects (specialized tools, science equipment, niche sports gear) have less training data → physics is more inventive.
How to spot it: object in shot is uncommon.
6. Clip is too long for stable physics
Like other drifts, longer clip = more accumulated physics error.
How to spot it: physics fine at start, fails by end of long clip.
Shortest path to fix
Step 1: Reframe the shot to avoid the physics
The single best fix: don’t ask the model to do physics it can’t.
# Replace physics-heavy shots with surrogate shots
# Bad — direct physics
"water pouring from kettle into cup, water level rising"
# Good — implied physics via cuts
1. Hand tilting kettle (no water visible in flow)
2. Cut to cup with water level high (already filled)
3. Add "water pouring" sound effect for sensory connection
# Bad — direct collision
"ball bounces off wall and rolls"
# Good — surrogate
1. Ball flying toward wall (just before contact)
2. Cut — sound effect of impact
3. Ball already rolling awayStep 2: Generate static-state shots, edit in motion
For impossible-to-render physics:
1. Generate "before" state (full kettle held over empty cup)
2. Generate "during" state (water mid-pour, no realistic motion required)
3. Generate "after" state (kettle put down, cup full)
4. Edit them together with sound designThis is a workaround pros use even on platforms with good physics.
Step 3: Use reference video where tool supports it
If you have real footage of similar physics:
# Runway Motion Brush / Pose tracking
- Some tools let you trace motion from a reference video
- The model follows the path you traced, not "physics imagination"
# This bypasses the physics-prediction stepStep 4: Shorten clips
# Cap clip length
- Physics issues compound; 2-3s is the sweet spot
- 5s+ guaranteed to have visible physics drift on hard casesStep 5: Switch to a stronger physics model
# Strongest on physics (as of 2026)
- Sora (best on water, basic dynamics)
- Veo (Google, similar)
- Kling 1.6 (improving)
# Weaker
- Pika 1.x
- Older Runway versions
# Note: even the best fail on complex physicsStep 6: For commercial work, composite real physics into AI footage
# Hybrid workflow
1. Generate the AI footage without the problematic physics
2. Shoot or source real footage of the physics (water, fire, etc.)
3. Composite real physics over AI background in After Effects / Fusion
4. Use rotoscoping or masks to integrateThis is the gold-standard approach for ads, music videos, and any high-stakes physics scenes.
Prevention
- Maintain a list of “physics shots the model handles” and “shots to avoid”
- For physics-heavy needs, plan a cut-based workaround upfront, not in production
- Always test physics shots with a stand-in / proxy before committing
- For high-stakes work, plan to composite real physics elements over AI background