When two solids are placed on the top of each other, they do not mix? Why?

Hint: We know that a solid is a state of matter characterized by particles arranged such that their shape and volume are stable. The constituents of a solid tend to be packed together so much closer than the particles in a gas or liquid. Solids cannot flow.

Complete answer:
The particles of solid are closely packed to each other. The atoms in a solid tend to attract each other, more than they are attracted by another substance.

So, atoms from one solid will not get caught up in the atomic bonding structure of another solid just by placing them together and so they do not mix.

There are strong electromagnetic forces holding the solids particles together so they cannot leave their positions. And get blend with other solid substance. Since the molecules are tightly bound, their kinetic energies are comparatively lower than the liquids and gases. This feature helps them in resisting the actions of forces acting externally

So, when two solids are placed on top of each other they do not mix because they have different boundary and a fixed shape and volume.

Note: One should keep in mind that solids can be mixed but not in general conditions. There are different techniques where we can mix them only when they are divided to a certain degree of homogeneity or mixedness that can be achieved by different operating conditions like high pressure and temperature.