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Video abstract for the article 'Scaling of viscous dynamics in simple liquids: theory, simulation and experiment' by L Bøhling, T S Ingebrigtsen, A Grzybowski, M Paluch, J C Dyre and T B Schrøder (L Bøhling et al 2012 New J. Phys. 14 113035). Read the full article in New Journal of Physics at http://iopscience.iop.org/1367-2630/14/11/113035/article. GENERAL SCIENTIFIC SUMMARY Introduction and background. When cooled below their freezing temperature many liquids do not crystallize but instead become supercooled and highly viscous. In this state these liquids are characterized by relaxation times and viscosities that increase dramatically by further cooling, in some cases by a factor of ten or more when temperature decreases by just a few percent. Likewise, increasing the density by applying an external pressure dramatically increases the relaxation time and viscosity. The most fundamental open question in the field of viscous liquids is: what controls the dramatic viscous slowing down? In general, this is a search for a physically justified function of two variables, temperature and density (or temperature and pressure). Main results. Based on the theory of isomorphs in dense liquids we show that for a particular class of liquids the relaxation time to a very good approximation is a function of a single variable, i.e., a scaling law is obeyed. The mathematical form of the scaling is uniquely determined by the interaction potential, but, notably, is not a simple power law. The scaling is demonstrated by extensive computer simulations of three model liquids, as well as for experimental results for two van der Waals liquids, dibutyl phthalate (DBP) and decahydroisoquinoline (DHIQ). Wider implications. Besides practical implications for simulation and experimental studies of viscous liquids, the scaling has important consequences for theoretical work; any general theory of the viscous slowing down of supercooled liquids must be compatible with the scaling.