WHAT is the most efficient shape for randomly packing things into a container? Physicists at New York University and Virginia Tech have carried out a series of experiments and, among all the shapes they have tested so far, the tetrahedron (a pyramid with four triangular sides) takes some beating. The researchers poured tetrahedral dice into containers which they shook until completely full. A magnetic-resonance imaging scanner was then used to see how tightly the dice packed themselves (pictured above). To quantify the packing efficiency, the researchers poured water into the containers to measure the amount of space between the dice. They report in the latest issue of Physical Review Letters that the tetrahedra filled roughly 76% of the available space in a large container, whereas experiments with spheres typically filled only 64% of the available volume. The physicists say the data will be useful when calculating, for example, how liquids seep through soils of different densities. They think the work might also have some use in packaging consumer products. Watch out for tetrahedral tomatoes.