Hmm, well the example with the water-filled cup is not bad, but maybe it's a little bit oversimplified. As mentioned earlier, the electrons are divided into shells with increasing energy the farther from the nucleus they get. What we haven't mentioned is that the electrons can only occupy specific energy levels. Between these shells of permitted energy, there are forbidden areas, with an energy that no electrons can have.

Ok, so to make the example with the water cup more realistic we would need to have a more unrealistic cup. It would be divided into compartments. The compartments can be filled with water which symbolizes the permitted energy levels. The barriers between the compartments are the forbidden energy levels and cannot hold any water.