A protein shaker is an enhanced plastic bottle that’s intended to go with you to the gym. It normally includes a very tight-fitting cap, a drinking spout of some sort, and most importantly, a mixer ball for the interior to help keep your drinks nice and smooth. Protein powder and other protein shake meal replacements often clump up if they’ve been sitting there for a bit, and the mixer ball is your ticket to revitalizing the smoothness of your shake.\r