I will explain why the counting invariants for sheaves on Calabi-Yau threefolds a la Donaldson-Thomas are equal to weighted Euler characteristics of the moduli spaces involved. Each sheaf, i.e., each point in the moduli space, contributes an integer weight to the sheaf count. In good cases this integer weight is related to the Milnor fibre of a Chern-Simons type function.