Étienne 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 Supérieure, and coauthor of Built on Sand: The Science of Granular Materials (MIT Press). José Bico is Associate Professor at ESPCI-PSL (Paris Science Lettres Université). Etienne Reyssat and Benoît Roman are CNRS researchers. All authors work at PMMH (Physics and Mechanics of Heterogenous Media) lab of ESPCI-PSL in Sorbonne Université.