What’s kept inside the prayer wheels?

The concept of the prayer wheel is a physical manifestation of the phrase "turning the wheel of Dharma" which describes the way in which the Buddha taught. Apart from the commonly used Mani wheel, which is rotated by hands, there are other types of wheels such as Water, Wind and Fire Wheels.

Traditionally, a prayer wheel is a cylindrical wheel on a spindle with the mantra Om Mani Padme Hum written on the outside of the wheel. At the core of the cylinder is a "Life Tree" with thousands and sometimes millions of mantras written on and wrapped around it. According to the Tibetan Buddhist tradition, spinning the prayer wheels have the same meritorious effect as orally reciting the prayers