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Try and remove the inner sliding mechanism, blocked by knobs you can only turn one at a time.
The most common variant uses a red plate and black knobs. There is a variant with black plate and blue knobs. This later version allows one to "reset" the puzzle.
There exists another variant named "Funny Ruler". See the fourth image.
If the puzzles state is viewed as a binary number the same "prime directive" applies as with "The Brain": A knob can only be moved if the knob immediately below it (to its right) is set to 1, and all other knobs below it are set to 0 (thus allowing the slider to move such that the next knob to be rotated can be positioned at the necessary location in the sleeve).
In 2001 Thinkfun (the successor of BinaryArts) made a version with blue knobs shown in image #5. This allows the knobs to be reset by flipping down the end and sliding out the bar.
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Thank you to the following people for their assistance in helping collect the information on this page: Joshua Bell, Andreas Nortmann, Brandon Enright.
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