Answer: When that couch is powered by an internal combustion engine. Or when you need a full leather race suit, gloves, boots and a helmet to sit on it. Or when it requires an attached front desk to serve as an aerodynamic element. Or when you need an airport runway to take it up to its full top speed.
Regardless of our reasoning, there is apparently a record being held for the World's Fastest Couch. If you're splitting hairs, we suppose there's nothing in the actual definition of the word couch that precludes the addition of an engine, and we also suppose the truly far-out-there among us could indeed use such a piece of furniture in his or her living room.
So, just how fast is the World's Fastest Couch? How's about 101 miles per hour? Yeah, that's fast... for a couch. Check out the video after the break to see the official record fall.