[header] How to clean the inside of a bottle [title] Fill your bottle less than ¼ full with gravel or rice. [step] If you choose rice, use raw, uncooked rice. If you choose gravel, choose a size in between sand and pea gravel. 
You'll want the gravel/rice to reach all the nooks, crannies, and corners of the bottle. [substeps] This method is great for irregularly-shaped bottles, as the gravel/rice will reach areas that may otherwise be hard to clean.