Pictures of (a)–(f) hierarchical grains and (g) rigid grains. The first six hierarchical grains are categorized by their drying or sintering conditions. Each image corresponds to the following granular matter: (a) wet, (b) dry, (c) sintered at 650 °C for 2 h (S650C02h), (d) sintered at 650 °C for 64 h (S650C64h), (e) sintered at 750 °C for 1 h (S750C01h), (f) sintered at 850 °C for 1 h (S850C01h), and (g) rigid glass beads.