We extend Brakke’s weak notion of mean curvature flow to have a free-boundary condition, and using toy examples we show why this extension is necessary. Contrary to the classical free-boundary flow, for which the barrier is ``invisible,'' our notion allows for the flow to ``pop'' or break up upon tangential contact with the barrier. We adapt Ilmanen's elliptic regularization and White's a priori curvature estimates to prove existence of the free-boundary Brakke flow, and smooth short-time existence.