Have you wondered why the mass properties for an assembly node sometimes show a negative number? This video will examine the philosophy of assembly-level operations like Promotions, Assembly Cut, Welds, Hole Series, etc. that can add or remove mass at the assembly level. Philosophically, the underlying original components remain unchanged, and the incremental additional positive mass as material is added in the assembled state (or the incremental removal of material while in the assembled state) can be represented as a positive or negative mass contribution at the assembly level. This video will demonstrate how we automatically take care of the mass properties "bookkeeping" in many cases. Enjoy!