It is a common macroscopic observation that knotted ropes or fishing linesunder tension easily break at the knot. However, a more precise localizationof the breakage point in knotted macroscopic strings is a difficult task. Inthe present work, the tightening of knots was numerically simulated, acomparison of strength of different knots was experimentally performed and ahigh velocity camera was used to precisely localize the site where knottedmacroscopic strings break. In the case of knotted spaghetti, the breakageoccurs at the position with high curvature at the entry to the knot. Thislocalization results from joint contributions of loading, bending and frictionforces into the complex process of knot breakage. The present simulationsand experiments are in agreement with recent molecular dynamics simulations ofa knotted polymer chain and with experiments performed on actin and DNAfilaments. The strength of the knotted string is greatly reduced (down to 50%)by the presence of a knot, therefore reducing the resistance to tension of allmaterials containing chains of any sort. The present work with macroscopicstrings revels some important aspects, which are not accessible by experimentswith microscopic chains.