The initial question is so easy: How big is the bucket? It is, however, deceptive in its simplicity. If the bucket is made out of steel or some other metal, its size and capacity are going to be fixed. A 5-gallon bucket is a 5-gallon bucket all of the time, and will carry a maximum of 5 gallons of liquid. Depending upon its density, however, the weight of 5 gallons of liquid can vary. Now, the question becomes more subtle: How much fluid can be moved from A to B in a 5-gallon bucket?? Depending upon the weight of the liquid and the strength of the carrier, the answer may vary. Perhaps the carrier can only lift a maximum of 50 lbs. If 3 gallons of a particular fluid weighs 50 lbs, that may be the maximum amount that can be moved in the 5-gallon bucket. Is that now its capacity? Then, would a lighter liquid change the capacity of the bucket to higher number (up to 5 gallons)? Now, what if the vessel was not a bucket, but a membrane of some type that was capable of expanding? The capacity might once again be dependent upon the weight and other characteristics of the fluid. Further, when stretched to its limit, the membrane may only remain intact for a few seconds before rupturing. Is the capacity of the membrane that amount of fluid in it for a few seconds before it bursts? Perhaps the capacity of the membrane is the maximum amount of fluid that can be retained in the membrane for an extended period of time. But, then, how much time describes a stable situation? Perhaps the issue is not so simple after all.