Illustration of the model
Use this diagram to explain what is implied by the normal model with constant error standard deviation.
Drag the sliders to show the flexibility of the most general model with arbitrary mean and standard deviation in each group. Take a few samples to show typical data from this model.
Finally click Equal st devn to constrain all groups to have the same spread of values.
You could mention that a nonlinear transformation of the response may make the assumption of equal spread in the groups more reasonable.