When an object gains energy, its particles speed up. As they speed up, they collide with more force. This increase in collision force causes the particles to bounce farther away from each other. When millions of these particles go through this process, the material increases in size – thermal expansion.
This can be easily summed up: as temperature of a substance increases, volume increases. There is one common exception to this rule. The overall thermal expansion of an object is a consequence of the change in the average separation between its constituent atoms or molecules.
When this phenomenon will affect things and applications in the real world, engineers must plan for this to occur and alter their designs accordingly.
Change in length = alpha x original length x change in temperature
[ L = 
Lo /\ T ]
Where alpha is the coefficient of linear expansion
These values are readily accessible. Here is one website with coefficients of linear expansion listed for common materials.
(source)