Boundary layer clouds play a leading role in determining climate sensitivity, especially in the Tropics. These clouds are highly parameterized in climate models, and are one of the largest sources of uncertainty in projections of future climate. This was pointed out by A. Arakawa and others as early as the 1970s and remains true today. The work presented here is an attempt to understand why the low cloud response to an idealized climate change varies among climate models. A simple aquaplanet setting with fixed sea-surface temperature reduces the complexity of the problem, but the produces essentially the same climate sensitivity as the full model. A suite of aquaplanet experiments with two independent climate models shows that low tropical clouds are of leading order importance for the climate sensitivity, and differences in the parameterized physics of the models leads to very different low cloud feedbacks.