Your skull is a closed compartment packed with brain, blood, and cerebrospinal fluid, with no air and no empty space for the brain to accelerate across. Every physical demonstration of the bouncing-brain model has to add air to work — and your head doesn't contain any.
What Did My Father's Mason Jar Demonstration Prove?
My dad is an engineer, and he enjoyed showing me things rather than telling me things.
One afternoon he took a mason jar and filled it with water all the way to the top. He dropped a raw egg inside. Then he took the metal lid and slid it flat across the surface of the water, sideways, so that no air was left underneath it. He sealed the jar. Inside there was now nothing but water and one egg. No air bubble at all, not even a small one.
He set it in front of me and said, "I'll give you five bucks if you can make that egg hit the inside of the jar."
I was eight. Five dollars was a lot of money, and this seemed like the easiest money I would ever earn. All I had to do was shake a jar hard enough to knock an egg against glass.
So I shook it. Nothing. I shook it harder. Still nothing. I started swinging it, snapping it back and forth, slamming it from side to side with everything I had. The egg turned and drifted inside the jar, but it would not touch the glass. Not once.
I got genuinely frustrated, because I was trying to make something happen that I was completely certain I could do. My dad stood there laughing at me the entire time.
When I finally gave up, he told me why it wasn't going to work. His explanation was one sentence.
**"Water doesn't get out of the way fast enough."**
Why Couldn't the Egg Reach the Glass?
Here is what he meant, spelled out.
For an object to travel across a space and strike a wall, it has to be able to build up speed on the way there. In an empty jar, that's easy. There's nothing in the way, so the egg accelerates freely until it hits the side.
In a jar filled completely with water, the egg cannot do that. Every bit of distance the egg travels toward the glass, it has to push an equal amount of water out of its path first. And water is not compressible. It cannot be squeezed into a smaller space to make room. It has to physically move somewhere else.
But in a sealed, completely full jar, there is nowhere else for that water to go. Every cubic inch inside the jar is already occupied by water. So the water in front of the egg has to travel around it, and that takes time. Long before enough water gets out of the way, the egg has already run out of momentum.
The same principle explains why sloshing works the way it does. When you shake a partly filled water bottle, the water can pile up at one end because there's empty space at the other end for it to move into. Take away that empty space, and there is no sloshing available. **A fluid can only slosh into a space that isn't already filled with fluid.** In practical terms, that means it needs air.
What Happened When He Poured Some of the Water Out?
My father wasn't finished. After he explained it, he opened the jar and poured out some of the water, leaving the rest of the jar filled with air.
Then he handed it back to me.
This time the egg hit the inside of the glass almost immediately, and it hit hard. I barely had to move the jar at all.
That was the actual lesson, and it took me years to understand how important it was. He hadn't shown me one demonstration. He had shown me two, and the only difference between them was the air.
The full jar was a purely **hydrodynamic** environment — an object moving inside a space entirely occupied by fluid. The partly empty jar was a mixed environment, part hydrodynamic and part **aerodynamic** — an object moving partly through fluid and partly through open air. Those two environments behave according to different rules. In one of them, the egg cannot strike the wall. In the other, it strikes the wall easily.
Same jar. Same egg. Same amount of shaking. Completely different result, decided entirely by whether air was present.
What Does the Standard Concussion Model Claim?
That distinction is why I have never been able to accept the standard explanation of concussion.
The most common model goes like this. A head takes a hit. Inside the skull, the brain lurches forward, strikes the bone, rebounds, and strikes the opposite side. The damage happens where the brain makes contact with the skull. That model is taught in locker rooms, in clinical settings, and in military medical briefings, and it is almost always illustrated with a computer-generated animation showing the brain moving back and forth inside the head.
Now compare that description to the two jars.
Is Your Skull the Sealed Jar or the Partly Empty One?
The sealed one.
The inside of your cranium is a closed compartment, and it is completely full. Your brain occupies most of it. The remaining space is filled with cerebrospinal fluid and blood — roughly 150 millilitres of cerebrospinal fluid circulating at any given moment. There is no pocket of air anywhere inside that compartment, and no empty space for your brain to travel across.
Your brain also isn't a rock sitting in there. It weighs about 1,400 grams on a scale, but suspended in cerebrospinal fluid its effective weight drops to roughly 50 grams — buoyancy cancels something like 96% of it (Brandis, *Fluid Physiology*).
So the animation asks you to picture a heavy solid crossing an open gap. The reality is a nearly weightless object, packed in fluid, with nowhere to accelerate to.
It is the egg in the jar.
Why Does Every Demonstration Use a Partly Empty Jar?
Once you understand the difference between those two jars, you start noticing something.
When someone demonstrates the bouncing-brain model using an actual physical object instead of an animation, they reach for a jar. This happens constantly — in classrooms, in training sessions, in videos. There is a scene in the film *Concussion*, with Will Smith, where he picks up a jar to explain the mechanism to his girlfriend.
Look at how much water is in it. It's about two-thirds full.
I have not seen a single demonstration of this model performed with a jar filled completely to the top and sealed. Every one of them leaves air in the jar.
I want to be fair about why they do this, because I don't think anyone is being dishonest. They are trying to make the movement visible. A demonstration where nothing appears to happen is a bad demonstration, and adding air makes the object move dramatically so the audience can see the point being made.
But adding air does not simply make the demonstration easier to see. **It changes the physics being demonstrated.** By introducing air, they convert a hydrodynamic environment into a mixed hydrodynamic and aerodynamic one — a set of conditions that does not exist inside your skull. The demonstration becomes vivid and easy to follow, and it also becomes a demonstration of something that cannot happen inside your head.
The air is not a harmless simplification. The air is the only reason the object can build up speed at all.
The same substitution happens in the animations. To make a computer model show a brain accelerating and striking the inside of the skull, the model has to simulate an aerodynamic environment inside a space that is purely hydrodynamic. Once you know to look for that, you stop watching physics and start watching an illustration.
Doesn't Hydroplaning Work the Same Way?
It does, and you have almost certainly experienced it.
Hydroplaning happens at high speed, not low speed. When you drive fast through standing water, the water underneath your tire doesn't have time to move out of the way, so the tire rides up on top of it and never reaches the pavement. Slow down, and the water has time to escape from under the tire, and your tread makes contact with the road again.
Notice what that means. Going faster did not help the tire reach the surface. Going faster is precisely what kept it away.
Fluids behave that way generally. Speed doesn't help an object push through a fluid to reach a surface. Beyond a certain point, speed is what prevents it.
What Does This Mean — and Not Mean — About Brain Injury?
I want to be exact here, because this argument gets stretched further than I intend it.
I am not saying the brain cannot be injured. It absolutely can be, and those injuries are real, observable, and in need of direct treatment. Where the brain comes close to the skull, you can get increased pressure that spreads nerve tissue apart and produces shearing. Rotational forces — an uppercut, a fast twisting hit — can produce shearing as well; angular acceleration is widely treated as central to the diffuse injury pattern in concussion research (Rowson & Duma, 2012). In severe impacts we see pressure-related bleeding that shows plainly on imaging.
None of that requires the brain to fly across a gap and slam into bone. And none of it is what I'm questioning.
What I am questioning is narrower, and it matters more than it might seem. A model is a claim about **where** an injury is located. Location determines what gets imaged, what gets examined, and what gets treated. If the model points at the surface of the brain because of an animation that needed air to work, then the search is aimed by a picture rather than by physics — and a search aimed at the wrong place will come back empty no matter how carefully it's conducted.
That is exactly what tends to happen. The brain scan comes back normal. The person still has symptoms. And they get told that nothing is wrong.
Something is wrong. It may simply be somewhere nobody looked, because the model never sent anyone there — most often the [craniocervical junction](condition-concussion-and-mtbi.html), which sits just below where brain imaging stops and takes [far less force to injure than brain tissue does](post-how-much-force-injures-your-neck-versus-your-brain.html).
Symptoms That Never Matched the Explanation You Were Given?
If your recovery stalled after a head injury and the explanation you received never quite fit what you were experiencing, it may be worth asking whether the injury was located correctly at the start.
That question is where our work begins at Cerebral. If you'd like a real evaluation, we're here.
References
- Brandis K. Cerebrospinal fluid. *Fluid Physiology*. Medicine LibreTexts. https://med.libretexts.org/Bookshelves/Anatomy_and_Physiology/Fluid_Physiology_(Brandis)/03:_Water_Balance/3.06:_Cerebrospinal_Fluid
- Rowson S, Duma SM. Brain injury prediction: assessing the combined probability of concussion using linear and rotational head acceleration. *Annals of Biomedical Engineering*. 2012;41(5):873–882. https://pubmed.ncbi.nlm.nih.gov/23299827/