Many superstitions are related to luck, though these are often specific to a given culture or set of related cultures, and sometimes contradictory. For example, lucky symbols include the number 7 in Christian-influenced cultures, but the number 8 in Chinese-influenced cultures. Unlucky symbols and events include entering and leaving a house by different doors in Greek culture, throwing rocks into the wind in Navajo culture, and ravens in Western culture. Some of these associations may derive from related facts or desires. For example, in Western culture opening an umbrella indoors might be considered unlucky partly because it could poke someone in the eye, whereas shaking hands with a chimney sweep might be considered lucky partly because it is a kind but unpleasant thing to do given the dirty nature of their work. In Chinese culture, the association of the number 4 as a homophone with the word for death may explain why it is considered unlucky. Extremely complicated and sometimes contradictory systems for prescribing auspicious and inauspicious times and arrangements of things have been devised, for example feng shui in Chinese culture and systems of astrology in various cultures around the world.