Polymers are long chain-like molecules made up of smaller molecules called monomers. It is well known that polymer chains often tie themselves in knots, but it was not clear how this influenced the mechanical properties of the polymer. Klein and colleagues modelled what happens when a knotted polymer chain consisting of 144 carbon atoms was stretched. They found that as the ends of the chain were pulled apart, the knot gradually got tighter and tighter - reducing the number of carbon atoms in the knot - until the polymer chain ruptured next to the knot. According to their calculations, a polyethylene polymer breaks once the number of carbon atoms in the knot is reduced to 23.