A Trapped Holmes and Watson

After being locked in an impenetrable strongroom, a panicked Dr. Watson cries, "How are we to escape, Holmes?" Calmly, the sleuth produces a small knife and a lemon, which he carefully cuts into small pieces. He then sticks the pieces of lemon on the wall in the shape of a door, and with a push creates a doorway. An astonished Watson asks, "How on earth did you do that, Holmes?" "Lemon-entry my dear Watson, Lemon-entry!"