Sitting down, with a pen (or stylus) in your hand and waiting for an idea to happen isn't the best way to get your creative on. Distracting your inner monologue and letting your brain process thoughts in the background leads to better results.
The odd approach to multitasking seems to suggest that our subconscious minds can process problems while we're concentrating on something else, as the little mice that pull the levers inside our heads continue to ponder ideas while we distract our conscious selves with the vacuuming or picking bits of stuff off things.
The concept has been described as incubation [PDF], in which gems of little concepts for funny things to say on Twitter slowly germinate in the back of the mind before popping out as fully-formed ZINGERS for literally no one to retweet or favourite. [QZ via BI]