By definition solids have their own shape and volume, whereas liquids have only their own volume. Liquids take the shape of the container in which they are poured, solids do not do this. This happens because of the strength of the intermolecular forces that are present in both solids and liquids. In solids the intermolecular forces are extremely strong, whereas in liquids these forces are somewhat smaller. Thus the distance between molecules that compose a liquid is higher than in solids. Because of this the molecules in liquids have some degrees of freedom (they can rotate, vibrate and also translate one with respect to the other). This is why the liquids take the form of the recipient that contains them.
Plasticine although it can be modeled changing easily its shape, it has its own particular shape when left alone, shape that it keeps over time. When placed into a container it does not change its form on its own to take the container shape.