A mashup is the combination of two+ different sources (characters, videos, songs, etc) often to hilarious effect. Sometimes mashup creators, by juxtaposing elements of different fictional worlds, create commentary on each world by the simple act of the juxtaposition. Synthesizing two worlds into one is certainly an exercise in independent creative expression, but is it copyright infringement? Consider the rise of mashup t-shirts that combine imagery clearly drawn from pop/geek culture franchises, as well as homage shirts – single-world art that attempts to capture the essence of a character or franchise without falling afoul of copyright or trademark laws. These uses are often less plainly parody or critical fair uses – the traditional core of fair use – but can be highly transformative uses of the underlying works.