In the spirit of the positive mass theorem Min-Oo conjectured that, if we fix the geometry of the boundary, the standard round metric on the hemisphere is the only one with scalar curvature at least n(n-1). In this talk we will see how to construct counterexamples for n>=3 as it was proved in the paper "Deformations of the hemisphere that increase scalar curvature" by Brendle- Marques-Neves.