According to superstition, if you open an umbrella indoors you are literally asking for bad luck to "rain on you". One explanation comes from the days when umbrellas were used as protection from the sun; opening one inside was an insult to the sun god who would then curse you with bad luck. Another theory states that an umbrella protects you against the storms of life, so opening one inside your house insults the guardian spirits of your home (whom also protects you from the storms of life), causing them to leave you unprotected.