The researchers chose nano-scale particles because they would have lots of surface area. Rangari’s group had already figured out how to make the nanoparticles. First, they ground clean eggshells into a fine powder. That process was a lot like grinding wheat into flour. Next, they blasted the fine powder with ultrasonic waves. These are energy waves that have a frequency higher than audible sound. The force broke the powder down even further into teeny, tiny bits. Each was shaped like a stacked deck of cards and was 10 nanometers across. (A human hair is more than 350,000 times as thick.)