The key to proper coffee grinding is obtaining a fairly consistent particle size that’s appropriate to the brewing method. The smaller the particle size, the greater the surface area of bean exposed to the water, and the faster its contents are extracted. Too great a range of particle sizes makes it hard to control the extraction during brewing. Small particles may be overextracted and large ones underextracted, and the resulting brew can be both bitter and weak. The common propellor grinder smashes all the bean pieces until the machine is stopped, no matter how small the pieces get, so coarse and medium grinds end up containing some fine powder. More expensive burr grinders allow small pieces to escape through grooves in the grinding surfaces, and give a more even particle size.