Why do predicates like know embed both declarative and interrogative clauses, whereas closely related ones like believe only embed the former? And why do predicates like be happy embed declarative and wh-interrogative clauses but not polar interrogative ones? The standard approach to this issue has been to specify lexically for each predicate which type of complement clause it can combine with. This view is challenged by predicates such as be certain which embed interrogative clauses only in contexts with negative polarity. In this talk I propose, (i) a novel unified semantics for declarative and interrogative embedding and (ii) a theory where embedding is constrained by semantic considerations. The reason for the apparent unembeddability of an interrogative clause under a given predicate is that the interpretative component derives a trivial meaning for the sentence, i.e., a tautology or a contradiction. Such trivial meanings manifest themselves in unacceptability. Crucially, both the lexical meaning of the predicate and the polarity of the sentence as a whole affect the outcome.