It may also incorporate a natural or man-made spring as part of the construction. Tanks are a component of an old history of collecting and storing local rainfall and water from streams and rivers for later use, mostly for agriculture and drinking water, but also for holy bathing and ritual. Modern tanks are used for storage of water and wastewater. The word "tank" comes from the Latin tectum, which means "a cover or roof." In architecture, a tank is a large container used to store or hold liquids. They can be either freestanding (as in our swimming pools) or part of a larger building (as in our water towers). In science, technology, and warfare, tanks are large fighting vehicles used for land warfare. They combine the mobility of a vehicle with the firepower and protection of a fortress. Tanks have been around since the early 20th century, but they become popular after World War II when they are adopted by many countries. Today, nearly all modern tanks are based on designs created by the Germans during World War II. The Russians also create their own tanks today, but they usually copy the German design. Modern tanks are divided into three main categories: heavy tanks, medium tanks, and light tanks. Heavy tanks are very powerful and expensive, while light tanks are fast but weak.