Heap of trouble

In my AP Biology class, we did a lab that involved extracting chlorophyll from spinach leaves. The first step was to grind the leaves into a paste with a bit of solvent, which we did the old-fashioned way using a mortar and pestle. The instructions said to add a "small amount of fine sand" to the leaves (to act as an abrasive, facilitating the breakup of the tough cell walls), and one of my…