Expansion joints are spacesbuilt into structures'
a. to prevent damage from thermal expansion.
b. to prevent thermal expansion.
c. to stop particles from moving.

to prevent damage from thermal expansion

Thermal expansion is the tendency of a material to alter in shape, area, and volume in reply to a variation in temperature. Temperature is a monotonic function of the average molecular kinetic energy of a material. When a material is heated, the kinetic energy of its molecules rises. An expansion joint or slip joint is an arrangement intended to carefully absorb the temperature-induced expansion and contraction of construction bodies, to absorb vibration, to hold pieces collectively, or to allow drift due to ground settlement or shakes. They are generally found within segments of buildings, bridges, footpaths, line tracks, piping systems, boats, and other structures.

Construction fronts, concrete pieces, and pipelines expand and contract due to warming and cooling from seasonal change, or due to other heat causes. Before expansion joint gaps were built into these structures, they would fracture beneath the stress induced.

a "theory" a suggestion that is not 100 % accurate, or a figure. some people say its a theory because they of all the passed theory's about one subject. it depends sometimes theory might be accurate and sometimes they are.