"Don't do that, Joseph!" I protest. "Don't let the blocks fall like that! They'll get chipped."
"I didn't!" he argues, staring at the pile at his feet.
"Do-not-let-the-blocks-fall-like-that!" Jason repeats, swooping a shelf of blocks onto the floor. I know what he wants me to do. It is what everyone in the block area wants: to repeat, in the same voice, my words to Joseph.
"Don't do that, Jason! Do not let the blocks fall like that! They'll get chipped."
Satisfied, Jason resumes his helicopter play. Joseph somehow feels validated, and all who witnessed the scene are content. No one else, right now, needs to find out if I am ambivalent about the rule. Nor do they need to worry if someone will get punished for breaking the rule. Every event contains the answer for some child's unasked question.
--The Boy Who Would Be a Helicopter, Vivian Gussin Paley, p. 107
There's a phenomenon I've come to terms with while working at the daycare. It can happen two different ways, but I'm sure that the reasoning behind both are the same.
Case 1 -- Independent. A child performs the same task continuously until mastery. For example, putting blocks through a shape sorter, or even dropping a spoon on the floor for Mommy to pick up.
Case 2 -- Social. A child will watch a peer perform a task and receive commentary from an adult, then will perform the task himself. See the quote above.
I believe that both of these cases occur because the child is checking for consistency. He needs to interact with his environment and cause an event to occur multiple times in order to fully understand what is happening.
Either event is difficult for a disconnected adult who has already mastered what the child is struggling with. Unfortunately, most of my coworkers fall into this category. At times it can be difficult for me, as well, until I remember to observe and follow.
Edit (4/2013): Here is a little about repetition in the Montessori classroom from Teaching From a Tacklebox.