A pon is a body of water smaller than a lake. In the United Kingdom, where the charity Pond Conservation has made some of the most extensive studies of ponds, the now widely adopted definition of a pond is 'A man-made or natural waterbody which is between 1m2 and 2 hectares in area, which holds water for four months of the year or more'. In other parts of Europe some biologists prefer to set the upper size limit at 5 ha , and in North America even larger waterbodies are often called ponds. The international Ramsar wetland convention sets the upper size limit for ponds as 8 ha. Although the size cutoff between ponds and lakes is partly subjective both are formed by ponding water. Ponds may be man-made or natural in origin and can be made by excavating a hollow in which water may lie, filling an existing depression with ground or surface water or by retaining water from a stream, or by forming a dam to impound the water in a valley. Ponds can be made by a very wide range of natural processes, although in many parts of the world these are now severely constrained by human activity. In some countries backyard ponds or garden ponds are popular and common.