Physics question - Two source patterns

"The only practical way to produce visible two-source interference patterns with light is, in effect, to derive two sources from a single source. Explain why this is so and describe one method of achieving the two sources."

I'm having trouble with this question. For the 'Explain why this is so' part, is it something to do with the frequency of the wave? Or maybe the wavelength? I'm not sure. If anyone could perhaps put me in the right direction without giving me the answer that'd be great.

Thanks in advance