We present a simplified model of shear thickening in concentrated colloids. The key idea of the model is that the trajectories of close approaching particles be simultaneously consistent with both the stress and the shear rate of the suspension. The weak shear thickening of Brownian hard spheres is reproduced. For coated particles it is predicted that shear thickening can be strong when, as particle separation decreases, dissipative coat-coat interactions are felt before conservative repulsion. A large compilation of data for the onset of shear thickening is also explained.