Processes A fluid is something that flows.
Fluids tend to conform to the shape of a container.
On Earth's surface, liquids tend to fill the bottom of an open or closed container and gases tend to fill closed containers. Unifying Concepts and Processes Rheology is the scientific study of the deformation and flow of matter. Unifying Concepts and Processes Joule heating occurs when electric current flows through a material. This is how an electric toaster works . The critical point is the temperature at which the differences between liquids and gases disappear.