Numerous technological advances have been made between the present day and the 31st century. The Head Museum, which keeps a collection of heads alive in jars and was invented by Ron Popeil (who has a guest cameo in "A Big Piece of Garbage"), has resulted in many historical figures and current celebrities being present, including Groening himself; this became the writers' device to feature and poke fun at contemporary celebrities in the show. Several of the preserved heads shown are those of people who were already dead well before the advent of this technology; one of the most prominent examples of this anomaly is Earth president Richard Nixon, who died in 1994 and appears in numerous episodes. The Internet, while being fully immersive and encompassing all senses—even featuring its own digital world (similar to "Tron" or "The Matrix")—is slow and largely consists of pornography, pop-up ads, and "filthy" (or Filthy Filthy) chat rooms. Some of it is edited to include educational material ostensibly for youth. Television is still a primary form of entertainment. Self-aware robots are a common sight, and are the main cause of global warming due to the exhaust from their alcohol-powered systems. The wheel is obsolete (no one but Fry even seems to recognize the design), having been forgotten and replaced by hover cars and a network of large, clear pneumatic transportation tubes.