Uncountable Optical Illusion

How many sticks are in this optical illusion?

Where does the thirteenth stick come from?

See Another Random Optical Illusion

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Explanation

In this optical illusion, when there are twelve sticks, it looks like they are all the same length. They really aren't. Note that the third stick from the right never has a bottom piece. Also note that the stick on the left has no top part when there are thirteen sticks. And when there are thirteen, the empty space is filled in with a stick that has no bottom part.