Why does DNA fluoresce under UV light?

[dae]

I read that it is the DNA in body fluids that make them fluoresce under UV light but I'm not sure why. If that's true, why do we need to add ethidium bromide to visualize DNA gels, wouldn't the DNA be visible by itself? Is it just a mater of intensity? I also heard that body fluids have to be dry in order to fluoresce, is that true and why would that be the case?

Thanks!

[CXCR4]

Wow that seems like that would be cool , do you mean like a black light fluoresce.

[mith]

EtBr makes it fluoresce more.