Buildings serve several needs of society – primarily as shelter from weather, security, living space, privacy, to store belongings, and to comfortably live and work. A building as a shelter represents a physical division of the human habitat (a place of comfort and safety) and the outside (a place that at times may be harsh and harmful). The largest parks can be vast natural areas of hundreds of thousands of square kilometres (thousands of square miles), with abundant wildlife and natural features such as mountains and rivers. In many large parks, camping in tents is allowed with a permit. Many natural parks are protected by law, and users may have to follow restrictions (e.g., rules against open fires or bringing in glass bottles). Large national and sub-national parks are typically overseen by a park ranger or a park warden. Large parks may have areas for canoeing and hiking in the warmer months and, in some northern hemisphere countries, cross-country skiing and snowshoeing in colder months. Factories arose with the introduction of machinery during the Industrial Revolution when the capital and space requirements became too great for cottage industry or workshops. Early factories that contained small amounts of machinery, such as one or two spinning mules, and fewer than a dozen workers have been called "glorified workshops". A bridge is a structure built to span physical obstacles without closing the way underneath such as a body of water, valley, or road, for the purpose of providing passage over the obstacle. There are many different designs that each serve a particular purpose and apply to different situations. Designs of bridges vary depending on the function of the bridge, the nature of the terrain where the bridge is constructed and anchored, the material used to make it, and the funds available to build it.