Solid is one of the four fundamental states of matter (the others being liquid, gas, and plasma). It is characterized by structural rigidity and resistance to changes of shape or volume. Unlike a liquid, a solid object does not flow to take on the shape of its container, nor does it expand to fill the entire volume available to it like a gas does. The atoms in a solid are tightly bound to each other, either in a regular geometric lattice (crystalline solids, which include metals and ordinary water ice) or irregularly (an amorphous solid such as common window glass).
The branch of physics that deals with solids is called solid-state physics, and is the main branch of condensed matter physics (which also includes liquids). Materials science is primarily concerned with the physical and chemical properties of solids. Solid-state chemistry is especially concerned with the synthesis of novel materials, as well as the science of identification and chemical composition.
The plural is one of the categories of grammatical number in many languages. Plural forms of nouns typically denote a quantity other than the default quantity represented by a noun, which is generally one (the form that represents this default quantity is said to be of singular number). Most commonly, therefore, plurals are used to denote two or more of something, although they may also denote fractional, zero or negative amounts. An example of a plural is the English word cats, which is derived from the singular cat.
Plurality is a linguistic universal, represented variously among the languages as a separate word, an affix, or by other morphological indications such as stress or implicit markers/context.
Words of other types, such as verbs, adjectives and pronouns, also frequently have distinct plural forms, which are used in agreement with the number of their associated nouns.
Some languages also have a dual (denoting exactly two of something) or other systems of number categories. However in English and many other languages, singular and plural are the only grammatical numbers, except for possible remnants of the dual in pronouns such as both and either.