Why does a cup of mixed colored sand un-mix when you shake it?
After you watchWhy does a cup of mixed colored sand un-mix when you shake it?
The short answer
A cup of mixed sand un-mixes when you shake it because the grains are different sizes. Each shake jiggles the pile and briefly opens tiny gaps between the grains, and the small grains slip down through gaps the big grains are too large to fit through. So the small grains sink and the big ones rise, and the mixture sorts into layers instead of blending.
Try this next
- What if you made the two bead sizes almost the same? Nudge the size slider so the big and small beads are nearly equal, then guess before you shake: will it still sort into layers, or stay jumbled? Watch how the layers fade as the gap between sizes shrinks.
- What if you only shook it a few times instead of a lot? Predict how many shakes it takes before you can see a layer, then tap shake one tap at a time and count. See whether the small grains trickle down a little with every single shake.
Now you — bend it
- What if What if the big and small beads weighed exactly the same? Would the jar still sort?Sorting is about who fits through the gaps, not who is heavier. Same size means no grain can slip past its neighbors, so weight alone will not make it sort.
- What if What if you slowly shrank the size gap between the two beads until sorting stopped?There is a point where the small beads are no longer small enough to fall through the gaps the big ones leave. Find the size ratio where the layers just barely refuse to form.
Can you prove it?Size, not weight, is what makes a mixture sort itself when you shake it. — Set both jars to the same grain size and shake hard — it stays jumbled. Then make one bead clearly bigger and shake again — now it sorts. The only thing you changed was size.
Design your own test:Pick how big the gap between the two grain sizes should be, then predict: a wide gap, a tiny gap, or no gap — which one sorts fastest, and which one never sorts at all?
Explain it to a 6-year-old: When you shake the jar, the little bits sneak down through the tiny holes the big bits leave, so the little ones go to the bottom and the big ones float up.
The whole story
How it works
When you shake a pile of grains, the whole pile lifts and loosens for a split second each shake, opening little gaps between the grains. If the grains are different sizes, the small ones can fall through gaps that the big ones are too large to pass, so the small grains keep trickling toward the bottom while the big ones get nudged upward. Repeat that over many shakes and the mixture separates into a layer of small grains below and big grains on top. If every grain is the same size, no grain is small enough to slip past its neighbors, so shaking only jumbles them and they stay mixed.
What people get wrong
People assume shaking always mixes things together better. It often does, but only when the pieces are similar in size. When a mixture has both big and small pieces, shaking does the opposite: the small pieces sieve down through the gaps and the big pieces ride up, so shaking sorts the mixture into layers instead of blending it. A size difference, not the shaking itself, is what decides whether you mix or sort.
The catch
Sorting by shaking is useful: machines shake gravel and seeds to line them up by size, and it is why cereal settles so neatly into the box. The catch is that it is also why crumbs sift to the bottom and the big nuts crowd the top, so the last handful is never like the first. Same-size grains avoid this and stay evenly blended no matter how hard you shake, but then shaking can never sort them for you when you want it to.
Questions kids ask
Why do the big nuts always end up on top in a can of mixed nuts?
Shipping and handling shake the can over and over. Each shake opens tiny gaps that the small nuts and crumbs can slip down through, but the big nuts are too large to fall through those gaps, so the small pieces sink and the big ones get pushed to the top. It is the same sorting that splits a jar of mixed sand into layers.
Does shaking ever actually mix things?
Yes, when the pieces are about the same size. With no grain small enough to slip past its neighbors, shaking just tumbles them around and they stay blended. Shaking only sorts a mixture when the pieces are clearly different sizes.
Do the small grains sink because they are heavier?
No, it is mostly about size, not weight. The small grains sink because they fit through the gaps that briefly open between the bigger grains. In fact even small light grains will sieve down past big ones, because the gaps decide who can slip through, not the weight.
How do you mix big and small things back together then?
Stirring or folding works far better than shaking, because it physically carries grains across the layers instead of letting them sieve. Adding a liquid also helps, since it floats the grains and stops them from settling through the gaps.
Talk about it
- Before we shake the trail-mix bag, guess: which pieces will end up on top, and why?
- If the small pieces sink, does that mean they are the heaviest? What else could decide who slips down?
- Can you think of a machine or a kitchen job where shaking things into size order would actually be useful?
For grown-ups
This is granular size segregation, famous as the Brazil-nut effect. Vibrating a mix of grains makes the packed bed dilate slightly each cycle; small grains percolate into the voids that open beneath larger grains (a sieving effect), and gentle convection rolls in the shaken bed carry large grains up the middle and trap them at the top. The net result is that larger particles rise. It is why muesli, mixed nuts, and gravel sorters all separate by size when jostled, and why shaking can un-mix a mixture rather than mix it.
Keep going
What else makes you wonder?
- If small grains sieve down through the gaps, why do the big nuts end up on top and not buried in the middle?
- What would happen if you shook the jar sideways instead of up and down?
- Could you ever shake a mixture so hard that it stays mixed instead of sorting?