Stirring rods are typically sterile glass tubes that are used to mix two substances together and to speed up a chemical reaction.
to stir liquids and other mixtures
to stir liquids
Is "stirring" too obvious an answer? It's also sometimes used for decanting a supernatant liquid or scratching the side of a vessel to induce crystallization, but stirring is the main one.
Ah, the humble stirring rod; most often used as a stirring apparatus. In other words, the stirring rod is mostly used for stirring things.