The mystery of sand.

XKCD’s Randall Munroe, writing now for The New York Times, explores a scientific mystery more baffling than quantum physics – what makes sand feel softer or harder:

If you build an hourglass and fill it with sand grains with a known range of sizes and shapes, there is no formula to reliably predict how long the sand will take to flow through the hourglass, or whether it will flow at all. You have to just try it.

Karen Daniels, a physicist at North Carolina State University who studies sand and other granular materials — a field actually called “soft matter” — told me that sand is challenging in part because the grains have so many different properties, like size, shape, roughness and more: “One reason we don’t have a general theory is that all of these properties matter.”

Our failure to find a general theory of sand isn’t for lack of trying. For everything from agricultural processing to landslide prediction, understanding the flow of granular materials is extremely important, and we just aren’t very good at it.

“People who work in particulate handling in chemical engineering factories can tell you that those machines spend a lot of time broken,” Dr. Daniels said. “Anyone who’s tried to fix an automatic coffee grinder knows they get stuck all the time. These are things that don’t work very well.”