Subject: How does a cell 'know' what exons to use?

I'm getting interested in how genes work and I've read several books
which explain that a gene is not a continuous strand of DNA, it is
actually spliced together by the cell out of many exons. Sometimes
the editing work done by the cell seems to be rather advanced - using
the same set of exons to create several proteins, by editing out one
or more of the exons. I've found no explanation of how the cell 
"knows" how to do this and I'd be grateful if you could explain this.
To me it seems that this splicing and editing done by the cell
contradicts the often heard statements that "it's all in the DNA" and
that "the DNA controls everything for its own benefit". Do you agree?

Re: How does a cell 'know' what exons to use?