Different kinds of organisms interact with each other in a number of different ways. In nature a relationship between two kinds of organisms that lasts over a period of time is called symbiosis. When a relationship between two kinds of organisms benefits both of them, it is called mutualism. For example, the yucca moth pollinates the yucca flower, and the yucca flower provides a place for yucca moth eggs to develop. A relationship in which one kind of organism lives on or in another organism and may harm that organism is called parasitism. The organisms parasites feed on are called hosts. A parasite of dogs and cats is fleas. A relationship where one organism benefits from another without helping or harming it is called commensalism. Certain tropical fish live unharmed among the poisonous tentacles of sea anemones, which provide safety for the fish.