Hi You say that these nouns are 'uncountable', but this is not so. You use the word simplicity when referring to a single simplicity and simplicities is used for two or more simplicities, the same with difficulty and difficulties - they are simply singular and plural nouns. For example 'the difficulty I have with English is the spelling' or 'the difficulties I have with English are the syntax and the spelling' (more than one difficulty) The fish question is one of those strange anomolies that appear in the English language to confuse us! These are some exceptions to the normal rule one fish, two fish one sheep, two sheep one deer, two deer one goose, two geese .... and yet ... one mongoose, two mongooses one mouse, two mice one cactus, two cacti one hippopotamus, two hippoptami ... and yet ... one duck billed platypus, two duck billed platypuses Katy