Nothing is hidden except in order to be revealed, nor does anything become concealed but that it might come into the open. - Mark 4.22
Mark was writing about Jesus' parables, not Interactive Fiction, although since according to at least some commentators Mark treats Jesus' parables as riddles and according to Nick Montfort (Twisty Little Passages. Cambridge, MA & London: MIT Press, 2003) the riddle is one of the precursors of IF, there may be a tenuous link here. It may be that Mark portrays Jesus' parables as employing concealment as a strategy of revelation; it is certainly the case that IF authors often hide objects in their games with the intention that the player will find them (hopefully with more success than the disciples in Mark).
There are various ways objects can be hidden and made to appear in response to a player action. We'll first explore how this can be done using Classes and concepts we've already met, and show how the library classes Hidden and PresentLater can help with the task.