Mayonnaise is a staple of almost every household, yet the reason why it’s so thick and viscous is a long-standing unsolved problem in physical chemistry: why does adding egg yolk to a runny mixture of oil and water increase the viscosity a thousand-fold? And, more generally, why does the viscosity of all solutions exhibit an unpredictably large increase in viscosity when a large amount of solute or surfactant (such as egg yolk) is added?