When you give information to another person and you think that this person should already know this information, you can use an obviousness declarative sentence. For example, Joan and Núria are talking about a common friend, Maria. Núria explains to Joan that Maria is pregnant. Joan asks Núria who is the father. Núria thinks that Joan should know this information because everybody knows that the father is Jaume, his partner.
Joan: De qui és, el fill?
Núria: D’en Jaume!