Which tool is best used for grinding solid materials into a fine powder?

The pestle is an essential tool used for grinding solid materials into a fine powder. It is commonly used in conjunction with a mortar, where the solid substances are placed, and the pestle is utilized to crush and grind them manually. This process allows for the transformation of larger particles into smaller, finer ones, which is particularly important in various biological and chemical applications, such as preparing samples for analysis or creating powdered forms of herbs and spices.

Other tools mentioned, such as the funnel, eyedropper, and beaker, serve entirely different purposes and are not designed for grinding. A funnel is typically used for transferring liquids or powders without spills, an eyedropper is used for dispensing small amounts of liquid, and a beaker is primarily a container for mixing or holding substances. Thus, the pestle stands out as the appropriate choice for the task of grinding solid materials.