Conversation so far:
What is progressiveness?
Progressive rock (or art rock) grew out of the classically-minded strains of British psychedelia.
What is psychedelia?
fused elements of cultivated music with the vernacular traditions of rock.
How did the Beatles contribute to progressiveness?
is generally credited as sparking a musical craze for the sound of the instrument in the mid-1960s.

Question: What instrument did the Beatles spark a musical craze for?
Self-contained rewrite: What instrument did the Beatles spark a musical craze for?