We propose a new technique to construct physical Zero-Knowledge Proof (ZKP) protocols for games that require a single loop draw feature. This feature appears in Slitherlink, a puzzle by Nikoli. Our approach is based on the observation that a loop has only one hole and this property remains stable by some simple transformations. Using this trick, we can transform a simple big loop, visible to anyone, into the solution loop by using transformations that do not disclose any information about the solution. As a proof of concept, we apply this technique to construct the first physical ZKP protocol for Slitherlink.