Pitcher plants (or pitfall traps) are carnivorous plants whose prey-trapping mechanism features a deep cavity filled with liquid known as a pitfall trap. Foraging, flying or crawling insects such as flies are attracted to the cavity formed by the cupped leaf, often by visual lures such as anthocyanin pigments, and nectar bribes. The sides of the pitcher are slippery and may be grooved in such a way so as to ensure that the insects cannot climb out. Through a mechanism of digestion, the prey is converted into a solution of amino acids, peptides, phosphates, ammonium and urea, from which the plant obtains its mineral nutrition (particularly nitrogen and phosphorus). Carnivorous plants occur in locations where the soil is too poor in minerals and/or too acidic for most plants to be able to grow.