Why do you think we are stupid?
How is that supposed to read?
a.) Why, do you think we are stupid? (Translation: Of course dummy, we all knew already)
b.) Why do you think we are stupid?
c.) Why do think we are
stupid? (Translation: Why do think we are stupid, as opposed to [insert adjective here])
In response to A, I was dispelling any belief in magic.
In response to B, no, I don't think that you are stupid.

In repsonse to C, I'm sorry if I come/came across as arrogant.
Diagram Errata Sheet Oops, I just noticed something, it would be impossible to drink from that cup, it's sealed shut! Just ignore that top flap of plastic and it all makes sense.-Penguin