Solids hold their shape and have a fixed volume. The molecules in solids are packed closely together and cannot be moved. Solids also have slow diffusion and low vapor pressure.
Solids are one of the three main states of matter; the others are liquids and gases. Matter consists of atoms, which are the smallest particles of matter. Unlike solids, liquids and gases do not hold their shape at room temperature. There are spaces between the atoms of liquids that continuously move. Liquids flow and take the shape of their container, and they are affected by gravity. Gases are always moving, and they have large gaps between their atoms. They take on the shape of their container and can be compressed into a smaller area.
Matter can change from one state to another. This change of state is called a phase change, and it is a physical change. Matter can be changed from one phase to another by altering temperature or pressure. Water is the only matter on earth that can be found naturally in all three states. Other matter changes also, but it usually only exists in two of the three states or requires the assistance of human technology to move through all three states.