Liquid is one of the principal states of matter. A liquid is a fluid that has the particles loose and can freely form a distinct surface at the boundaries of its bulk material. The surface is a free surface where the liquid is not constrained by a container. A liquid's shape is determined by the container it fills. That is to say, liquid particles (normally molecules or clusters of molecules) are free to move about the volume, but they form a discrete surface that may not necessarily be the same as the vessel. The same cannot be said about a gas; it can also be considered a fluid (things that flows, including gas), but it must conform to the shape of the container entirely.

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