At a pressure of 220 atm and a temperature of 374 °C there is a second-order phase transition between water and steam. Understanding it requires a key concept of both condensed matter and elementary particle physics: the renormalization group. Its basic ideas are explained with images from computer simulations of the lattice gas model. Then I briefly review how the renormalization group is used to compute critical coefficients for the water–steam phase transition. The results of this calculation are in good agreement with experiment. Finally, some applications in particle physics and string theory are mentioned.

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