My old student Camila Correa continues to revolutionise the basic fundamentals and assumed wisdom of galaxy formation. In particular she thoroughly explored the simple idea that infalling gas will shock against the other gas floating around the galaxy. In this paper, Camila used the EAGLE simulation series to explore the way in which exploding stars (supernovae) or feeding blackholes (AGN) impact that development of the hot halo. Essentially the supernovae ejects gas from the galaxy into nearby space, presenting a bigger target to infalling material, and hence makes the hot halo easier to form. The blackholes on the other are more energetic and eject material from the halo entirely, making it harder to form the hot halo in the first place. Overall, Camila showed that there was a critical halo mass above which the hot halo will form, around a half the size of the Milky Way at the present day.