The difference between helpful and potentially harmful enemas lies in the amount of water injected and the frequency of use. Using a cup or two of water to induce a bowel movement may eventually cause dependency, will not strengthen the colon and may after years of this practice, result in distention and enlargement of the rectum or sigmoid colon. However, a completely empty average-sized colon has the capacity of about a gallon of water. When increasingly larger enemas are administered until the colon is nearly emptied of fecal matter and the injection of close to a gallon of water is achieved, beneficial exercise and an increase in overall muscle tone are the results. Correctly given, enemas (and especially colonics) serve as strengthening exercises for the colon. This long tubular muscle is repeatedly and completely filled with water, inducing it to vigorously exercise while evacuating itself multiple times. The result is a great increase in muscle tone, acceleration of peristalsis and eventually, after several dozens of repetitions, a considerable reduction of transit time. Well-done enemas work the colon somewhat less effectively and do not improve muscle tone quite as much as colonics. Injecting an entire gallon of water with an enema bag is very impractical when a person is eating normally. But on a light cleansing diet or while fasting the amount of new material passing into the colon is small or negligible. During the first few days of fasting if two or three enemas are administered each day in immediate succession the colon is soon completely emptied of recently eaten food and it becomes progressively easier to introduce larger amounts of water. Within a few days of this regimen, injecting half a gallon or more of water is easy and painless. Probably for psychological reasons, some peoples' colons allow water to be injected one time but then "freeze up" and resist successive enemas. For this reason better results are often obtained by having one enema, waiting a half hour, another enema, wait a half hour, and have a final enema. A colonic machine in the hands of an expert operator can administer the equivalent of six or seven big enemas in less than one hour, and do this without undue discomfort or effort from the person receiving the colonic. However, the AMA has suppressed the use of colonics; they are illegal to administer in many states. Where colonics are legal, the chiropractors now consider this practice messy and not very profitable compared to manipulations. So it is not easy to find a skilled and willing colonic technician.