The simple symmetric exclusion process with a slow site consists of a system of independent random walks under the rule that each site can contain at most one particle and, additionally, a specific site behaves as a trap. We prove here that, as a function of the trap's strength, the system exhibits a phase transition. Depending on the parameter of the trap, the macroscopic limit of the system can be driven by the heat equation, the heat equation with certain Robin boundary conditions, or the heat equation with Neumann boundary conditions. The talk will be focused more on ideas than technical aspects. Joint work with P. Gonçalves and G. Schütz.