Here is a picture of me pouring some cocoa powder into a container.
I have turned the jar of cocoa powder upside down, yet the cocoa powder doesn't just fall out. I have to shake it a little for the powder to fall out. Then the powder stops falling out and I have to shake it some more. (Arguably a video would be more useful to demonstrate this than an image, but I think you guys know what I'm talking about)
Can somebody explain this phenomenon?
Gravity is obviously acting on the powder, so I would expect it to just fall out of the jar with little resistance. Why does it get "stuck"? Is it just so compact that the powder particles push against each other, and the particles on the "edges" push against the glass container, so I have to dislodge them by shaking?