Ever since it was shown that graphene—a single layer of carbon atoms—could be isolated from graphite, it has occupied a center stage of condensed matter physics. The popularity of graphene is rooted in the unusual nature of its low-energy excitations: near the Fermi level, the electron energies scale linearly with their momenta. This means that the electrons can be described as “massless” fermions, though with a velocity of about 300 times less than the velocity of light. The linear dispersion relation also implies a vanishing density of single-particle states at the Fermi level, which should make the effects of the Coulomb interaction between electrons weak. This usual mantra, however, may sometimes be quite misleading, as argued by Markus Muller at the ICTP in Trieste, Italy, Jorg Schmalian at Ames Lab and Iowa State University, US, and Lars Fritz at Harvard University, US, in a paper appearing in Physical Review Letters[1]. They show that a particularly suitable measure of how strongly the excitations in a given quantum fluid interact is given by the dimensionless ratio between the fluid’s sheer viscosity and entropy density. They find that the value of this ratio in graphene is surprisingly close to its likely lower bound [2]. Such a low viscosityto-entropy ratio, somewhat paradoxically, means that the electrons in graphene form a quantum liquid that is, in fact, strongly interacting. By this criterion graphene comes closer to being a “perfect fluid” than several other quantum systems that have often been labeled as strongly correlated. Landau’s notion of a Fermi liquid as a system of interacting fermions that, at low energies, effectively behave as noninteracting quasiparticles is the central paradigm of many-body physics. Our modern way of thinking about a Fermi liquid is to use the language of renormalization group theory—a theory that extracts the essential physics of many-body systems by zooming out from the microscopic details. In this framework, one would say that although the Coulomb interaction between electrons in a typical metal in an absolute sense is not weak, its effective strength depends on the energies at which the system is probed [3]. In a Fermi liquid, the effective interaction parameters decrease at lower temperatures and frequencies until they reach saturation. This idea receives its simplest realization precisely in graphene: The Fermi surface is shrunk to just two points (“Dirac” points) in momentum space, near which the energy depends linearly on the quasiparticle’s momentum and the density of quasiparticle states also vanishes linearly. The scarcity of low-energy excitations renders all the short-range components of the Coulomb electron-electron interactions irrelevant[4]. Using a term from renormalization group theory, these interactions “flow” towards zero as fast as the first power of temperature. From this perspective, graphene appears to be a perfect example of a weakly interacting Fermi liquid. Or so it would seem. What about the fact that the Coulomb interaction is a long-ranged force? In metals, this does not matter much, since the quasiparticles screen the interaction and make it effectively short ranged. But in graphene there are not enough low-energy quasiparticles to screen effectively, and the Coulomb interaction remains long ranged [5]. As a result, the Coulomb interaction does not change with the energy scale. Or, in the parlance of renormalization group theory one would say that the coupling g in the Coulomb interaction V(r) = g/r represents an exactly marginal coupling, which does not flow at all with the change in energy scale. Its main effect, it turns out, is to produce a shift in the Fermi velocity that diverges at low temperatures, albeit only as a logarithm: v(T) ∼ g log(T0/T), where the high-energy scale T0 ∼ 105 K is set by the width of the conduction energy band [6]. If one defines the dimensionless strength of the Coulomb interaction α(T) = g/(hv(T)), this coupling constant would slowly approach zero as the system is probed at progressively lower temperatures (Fig. 1). So theorists can paint a picture of graphene with only a few strokes: At temperature or frequency scales