I've realized Millikan's experiment to determine the electron's charge. However I have a very basic question that puzzles me. Why are some (maybe all) oil drops charged? Basically in the lab we had a very bit of oil and with a device we blow on the flux of oil that leaves a very thin capilar and this creates lots of very, very tiny drops. Why are some charged?!!! (I've realized that some are more charged than others and some are positively charged and some are negatively charged, due to conservation of charge I guess).