As I said yesterday, ser and estar can be really tricky to get to grips with for English speakers, as we use 'to be' for both. I get this wrong plenty of times, as do many much more fluent Spanish speakers. Some examples that really confuse me are where 'estar' is used for pretty permanent situations which I would expect to use 'ser' for, whilst some more transient things use 'ser'.
For example -
Está casado = He is married
Estoy vivo = I am alive
Both of these are generally considered to be pretty long term situations, if not totally permanent. But on the other hand
La comida es muy rica = The food is very good (the chances are it won't be for long though as it will be eaten)
So the rule that 'ser' is for permanent and 'estar' for transient can't be relied on. One final example
Está muerto = He is dead
This is about as permanent as anything gets, but despite this 'estar' is the correct verb to use. I still have no idea why.