chunk: Thin leaves are seized with the mouth, while thick ones are dragged by creating a vacuum. Leaves and stones are used to plug up the burrow. This may deter predators, keep out water and/or keep out chilled air (the latter is Darwin's preferred function). Leaves are dragged in mostly by the tips, which is the easiest way of doing it, but when the base is narrower the worms change behaviour. They drag pine needle clusters in by the base. Petioles are used to plug up burrows, and for food. Worms drag experimental triangles of paper by the apex most of the time, and do not rely on trial and error. Worms excavate burrows by consuming material or, preferably, pushing it away. They mainly consume soil for nutrients. They are found down to six or more feet, especially in extreme conditions. Burrows are lined, which serve several functions, and terminate in a chamber lined with stones or seeds. Worms are found all over the planet, some on isolated islands; how they got there is a mystery.