Etienne Guyon is Emeritus Professor at ESPCI (Paris Industrial Physics and Chemistry Higher Educational Institution), a fellow of the American Physical society and of the Institute of Physics, Honorary Director of the Ecole Normale Superieure, and coauthor of Built on Sand: The Science of Granular Materials (MIT Press). He works at the PMMH (Physics and Mechanics of Heterogenous Media) lab of ESPCI-PSL in Sorbonne Universite. Jean-Yves Delenne is Director of Research at INRAE (the French National Research Institute for Agriculture, Food and Environment).He is a pioneer in research on advanced numerical simulations of granular materials. Farhang Radjai is Director of Research at the CNRS (the French National Center for Scientific Research). He is a pioneer in research on advanced numerical simulations of granular materials.