The states of matter are the different physical forms matter can achieve depending on its molecular configuration. The three classical states — solid, liquid, and gas — are easily observable on Earth and can transition from one to the other. Among non-classical states of matter, plasma is the most abundant, accounting for a large percentage of the baryonic matter in the known universe. Other non-classical states can bear strong similarities to classical ones, require specific environmental conditions, or exist primarily in theory. The Bose-Einstein condensate, one of the earliest confirmed theoretical states, is widely accepted as the fifth primary state of matter, following the three classical ones and plasma.
An object's state of matter depends largely on the strength and character of its inter-molecular forces. In solids, for example, the forces are strong enough to pack particles closely together, permitting vibration as their only form of movement. As a result, solids are able to achieve definite shapes and volumes. Liquids, on the other hand, have weaker inter-molecular forces pulling the particles together. Liquids have no definite shape; they follow the form of their containers and fill as much space as their volumes allow. The inter-molecular forces of gases are even weaker than liquids, resulting in a shape that tends to expand throughout the entire container, regardless of the gases' volume.
The three classical states of matter can transition from one to the other with the aid of heat and pressure. The process through which solids become liquids is known as melting, while the reverse is known as freezing or solidification. This is often illustrated with water — in its liquid state, water can be frozen into ice, a solid, which can subsequently be melted back into liquid water. With enough heat and pressure, liquids can become gases through a process known as vaporization; the reverse of which is known as condensation. Solids can transition directly into gases via sublimation, while gases can become solids through deposition.
Although the classical states of matter are commonly observed on Earth, plasma makes up most of the matter in stars. Plasma is an ionized gas that, unlike the classical state, generates electromagnetic currents. Bose-Einstein condensates, named after Satyendra Nath Bose and Albert Einstein, are gases cooled to a point where the particles stop acting independently, resulting in a singular, frictionless quantum state. Outside of these five main states of matter, scientists theorize about numerous others, such as strange and dark matter. Glasses and liquid crystals, among others, are considered different enough from the five to be classified as non-classical states of matter of their own regard.