Complex sentence: Non-steady-state situations appear after an imposed change in temperature at a boundary, or inside an object, as a result of a new source or sink of heat suddenly introduced within an object, causing temperatures near it to change in time.
Split into simpler sentences: Non-steady-state situations appear after an imposed change in temperature at a boundary of an object. They may also occur with temperature changes inside an object, as a result of a new source or sink of heat suddenly introduced within an object, causing temperatures near the source or sink to change in time.