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Stringy

To solve this puzzle you must wind the string around the grey pegs in such a way that removing any single peg will allow the string to fall. Imagine the start and finish nodes pulling on the ends of the string: the string must come free when any one peg is removed, but must not come free if no pegs are removed.

Solution

author: unknown

The aim is as described in the body, we must simply wrap the string round the pegs such that removing either will let the string fall, similar to Borromean rings. It turns out there is a paper on this problem here which contains this problem as an example. Alternatively, group theory can be applied (see the superstring solution where we must solve it manually).