The acorn paradox

The Acorn Paradox is a cause and effect paradox that seems to suggest the creation of something from nothing. It proceeds as such:
A time traveler goes back in time a hundred years and plants an acorn. When he travels back to his own time, he finds an oak tree has grown in the spot. He takes an acorn from the new tree and goes back in time to a few moments after he planted the first acorn, digs it up, and plants the new acorn in its place. He puts the original acorn in his pocket and travels back again to his own time. Now, a new oak tree has grown in the spot of the original, but the original acorn is in his pocket. The new tree would appear to have grown from nothing.
The paradox features in the science fiction book series The Glimmering Time as the main plot device.
 
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