A degreaser is useful in removing greasy dirt from parts. Traditionally petroleum based products have been used. DO NOT use gasoline. Although it is an effective degreaser, its volatility (i.e. tendency to explode) make it a poor choice for shop work. If you must use a petroleum product, choose naphtha (usually available in hardware stores) or kerosene. A better choice is a modern, biodegradable water based degreaser. Specific blends for bicycle cleaning are available from bike shops and online retailers. You can probably save some money, though, by purchasing degreaser from an auto parts retailer. Whatever you use, after cleaning moving parts with a degreaser, always follow up with lubrication. All residual degreaser should be wiped off the parts before applying lubrication, as it would otherwise dilute the lubricant.