Things I thought I knew about sumo wrestling before I watched it:
It consists of fat Japanese men wrestling in nappies.
Things I found out from going to a sumo tournament:
Fat's not the word. Even the biggest sumo wrestlers (rikishi) live on a healthy-ish diet of 'chankonabe', or meat and vegetable stew. They just eat a lot of it. The result is that under their layer of fat rikishi are enormously muscular, which makes them surprisingly graceful - at least inside the ring. Judge for yourself, from the photos I took at the tournament:
The men aren't actually all that big. Some are pretty small. There are no weight divisions in sumo, which means that a small wrestler can sometimes find himself up against someone twice his weight. Eep.
Rikishi aren't always Japanese. Japan may be the only country where it's an official sport, but wrestlers of various nationalities compete.
It isn't just men pushing each other around in a ring: both the sport and the wrestlers' lives are highly regulated. Each little detail, from the wrestlers' hairstyles to the slippers that they wear, is chosen for them and steeped in tradition and ritual. Rikishi live together in 'stables' and on the rare occasions that they appear in public must wear full traditional dress of cotton yukata and wooden slippers (geta). Geta make a loud 'clip-clop' noise on the ground, and so one of the rewards of moving up the sumo ranks is that straw slippers can be worn and the wrestler can avoid sounding like a horse.
The outfit that wrestlers wear in the ring? Not (surprise surprise) a nappy, but a silk belt or 'mawashi'. Nowadays if a rikishi's mawashi completely unravels in a bout he is immediately disqualified - a rather ignominious end to the match. However, this is apparently due more to western awkwardness about nudity than Japanese tradition. In fact Japan is pretty OK with nudity, doubtless a result of the onsen culture.