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The butter lamps help to focus the mind and aid meditation. According to the Root Tantra of Chakrasamvara, "If you wish for sublime realization, offer hundreds of lights" 1.
Pilgrims also supply lamp oil to gain merit. The monks in the monastery manage the actual lamps, taking extreme care to avoid starting one of the devastating fires which have damaged many monasteries over the years. For safety, butter lamps are sometimes restricted to a separate courtyard enclosure with a stone floor.