Entity Hierarchy (parenting)
An entity with a parent will duplicate its parent's motion (move with it). An example would be parenting a dynamic light to a lamp prop so the light becomes part of the lamp and moves with it. A group of objects parented together form an hierarchy and essentially become one object.
To parent two objects together, type a name for the parent object in its Name property. For the other object, set its Parent property to that same name.
Only certain entities can have parents. For example, a prop_physics cannot because it is its own physics object. In this case, a prop_dynamic or prop_dynamic_override should be used instead.