"Tribute", "memorial", or "commemorative" hacks note the arrival, passing, or anniversary of some noteworthy person, tradition, institution, or idea (e.g. the 10th anniversary of Wikipedia). Another broad category of hacks contains strong elements of social commentary or street protest (e.g. "Nth Annual Spontaneous Tuition Riot") about events on campus or in the world at large. But the strongest element of many hacks is the sheer joy of conceptualizing something new, and then reifying it with effective engineering, both technical and social (e.g. installing a full-sized mockup solar-powered subway car on the parapet wall around the base of the Great Dome, and then "driving it back and forth under remote wireless control" from Killian Court, some five stories below, "after sundown").