three
o'clock
`` See ? Five minutes before I shall place it there , it _ is _ there ! '' His other colleague frowned at the cube . `` But , '' he said , `` what if , now that it has already appeared five minutes before you place it there , you should change your mind about doing so and _ not _ place it there at three o'clock ? Would n't there be a paradox of some sort involved ? '' `` An interesting idea , '' Professor Johnson said . `` I had not thought of it , and it will be interesting to try .