16 ignorant : a state of not knowing what a pronoun is , or how to find the square root of 27.4 , and merely knowing childish and useless things like which of the seventy almost identicallooking species of the purple sea snake are the deadly ones , how to treat the poisonous pith of the sagosago tree to make a nourishing gruel , how to foretell the weather by the movements of the tree-climbing burglar crab , how to navigate across a thousand miles of featureless ocean by means of a piece of string and a small clay model of your grandfather , how to get essential vitamins from the liver of the ferocious ice bear , and other such trivial matters .