It all starts with an .STL file (STereoLithography), which you can download for free on Thingiverse or design on Tinkercad. An .STL contains instructions on how to print a design in three dimensions. This file is loaded onto a 3D printer, which heats its nozzle up to about 180°C. PLA filament feeds through the nozzle and melts onto the printer bed to build an object layer by layer. PLA filament is a biodegradable, non-toxic plastic made mostly of cornstarch.