In a pure hard sphere system dynamics are arrested due to repulsive caging. However, changing the interaction potential from hard sphere-like to slightly attractive (e.g. by adding a non-adsorbing polymer in order to induce depletion) can melt the glass. This - at first glance counterintuitive - behavior can be explained as follows: If the particles attract each other they obviously prefer to stay even closer to each other leaving at the same time some free volume elsewhere in order to balance the overall volume fraction. This free space is eventually available to particles escaping their cages: The sample becomes ergodic or fluid-like. If one furthermore increases the interaction potential, the system again gets stuck. However, now the slowing down of the dynamics is dominated by attractive interactions: Nearest neighbors touch each other and form a connected gel-like network. Therefore this type of glass is called 'attractive'. The whole phenomenon is called reentrant glass transition and has been predicted by Mode Coupling Theory and measured by Pham et al. (Science 296, 104 (2002)) using light scattering techniques.