Hourglass of constant weight as an illustrative example of a system with a variable mass

We provide an alternative approach to a problem posed in the article “Hourglass of Constant Weight” by Volker Becker and Thorsten Pöschel, which appeared in Granular Matter 10 (2008), 231-232, i.e. 14 years ago. Our goal to return to this article after so much time is purely pedagogical. Although there is a slight and subtle inaccuracy in the derivation of equation (8) in the work of Becker and Pöschel, which we correct, we use this circumstance only to give a detailed pedagogical exposition of an excellent physical problem and to show the pedagogical usefulness of the concept of momentum flux when considering the mechanics of systems with variable mass.