A school in Denmark has set up a greenhouse on site, that contains a large pond supporting a freshwater ecosystem. The objective is to give the children, who have otherwise little contact with nature, an opportunity to work with practical ecology. Each class looks after the greenhouse for a month, feeding the fish, watering the plants, cleaning it, checking air and water temperatures, humidity, and water nitrate content. Two or three children are always responsible for looking after it. The greenhouse is being integrated into biology classes.