Imagine hefting a liter of water in your hands. That's a kilogram of weight. Divide that by a billion, and you've got a quantity called a microgram—a thousandth of a single milligram. Divide that amount by a billion, and you've got a femtogram —which it's almost impossible to get an intuitive sense for. But divide by a billion yet again , you've got roughly the mass of a single proton—and that's what scientists have measured with unprecedented precision in a surprising new experiment at the Max Planck Institute in Germany.
