Question

why can liquids flow, whereas solids can be heaped

Answer 1

Liquids always tend to flow, because the intermolecular forces between molecules is less. However in solids, the bonds are very strong and prevent molecules from moving around. This makes it easier for solids to be heaped, without losing their shape and structure. Liquids often are volatile and take the shape of the container, since the molecules are more free to move around.