Community rituals are usually based on traditional community celebrations found in folkloric collections by authors such as Marian McNeill, Kevin Danaher or John Gregorson Campbell. These celebrations often involve bonfires, dances, songs, divination and children's games. More formal or mystical CR rituals are often based on traditional techniques of interacting with the Otherworld, such as the act of making offerings of food, drink and art to the spirits of the land, ancestral spirits, and the Celtic deities. CR ritual structures are based on the ancient Celtic idea of the "Three Realms" - Land, Sea and Sky - with the fire of inspiration seen as a central, uniting force. Many CRs have altars and shrines to the spirits and deities they believe in. They may place these altars at outdoor, natural locations such as wells, streams, and special trees. Some CRs practice divination, such as the taking of omens from the shapes of clouds or the behaviour of birds and animals.