What makes sand to behave like a liquid when it is poured? I couldn't find a satisfying physics explanation of what the exact mechanism and phenomenon responsible for making the sand to behave like a liquid when poured although its particles are solids?
 A: In soil mechanics, the maximum shear stress, known as shear strength, the soil can endure without sliding is typically modeled by
$$
\tau = \sigma \tan \phi \ + \ c
$$
where

*

*$\tau$ is the shear strength

*$\sigma$ is the normal stress†

*$\phi$ is the shear friction angle, defined such that $\tan \phi$ is analogous to the coefficient of friction

*$c$ is called cohesion, a.k.a. stickiness

The ideal sand is characterized as having $0$ cohesion, and thus have $0$ shear strength when there is no normal stress acting on it. When you pour sand, it is "depressurized" from its own weight and can be considered to have $0$ normal stress, which implies it has $0$ shear strength.
Now, the basic distinction of fluids and solids is exactly that fluids can't sustain shear stress under constant strain, which is the same phenomena you see with pouring sand.

As an aside, when you pour sand, such as in an hourglass, the angle of the sand pile is exactly $\phi$, which is why it's defined as such.
† For dry soils only.
A: Sand constists of solid grains that are not firmly attached to each other. On the other hand, the grains have corners that make them not roll easily, or perhaps just friction that makes them not slide easily. You can see the same thing in a pile of gravel.
If you pour sand, the grains bounce around. They don't stick together. So each bounce allows them to move with respect to each other. They can move indepedently like drops of liquid.
On the other hand, there are many properties of a liquid they do not have.

*

*Drops of liquid do stick together.

*Liquid wets (sticks to) a surface.

*Liquid has surface tension

*Liquid flows without macroscopic bouncing, and typically has viscosity.

*Liquid has no grains larger than a molecule.

