The Hippo air roller, or Hippo roller, is a device used to carry clean water more easily and efficiently than traditional methods, particularly in the developing world and rural areas. It consists of a barrel-shaped container which holds the water and can roll along the ground, and a handle attached to the axis of the barrel. Currently deployed in rural Africa, its simple and purpose-built nature makes it an example of appropriate technology. The rollers cost around $125 each and they are mainly distributed by NGOs. Physical design The drum of the Hippo water roller is made from UV stabilized linear low-density polyethylene and is designed to cope with the rough surfaces found in rural areas. The drum's volume is 90 liters (about 24 gallons). It has a large opening (135 mm / 5.3 inch diameter) for easy filling and cleaning. The size of the opening was originally determined by the availability of a large enough lid. The lid then on its part determined the roll radius of the roller, since enough clearance was required with the lid fitted to keep the latter clear from the ground and any obstacles which might damage it. The lid was eventually further recessed on later models to enhance its protection. The steel handle allows the roller to be pushed or pulled over difficult and very rough terrain. The overall width of the roller with handle attached was determined by measuring the average width of a standard doorway and sized to allow it to be pulled through freely. This parameter together with the roll radius and the rounded shoulders eventually determined the average volume of 90 liters. The steel handle is fitted with special polymer end-caps to reduce friction and wear and prolong the life of the pivot cavities in the drum. During development a water filled roller was drawn behind a vehicle over a dirt road at 20 km/h for 15 km without any significant signs of wear on both the roller outer surface or pivot cavities. The roller is rounded at the shoulders to simplify tilting when wanting to pour from the full roller. However, the roller is also very stable in the upright position when it rests on a small, flat surface. The roller has hand grips at the bottom and top to make emptying the container easier. History The barrel, originally called "Aqua Roller", was the brainchild of two engineers