According to legend, the game was created in 1823 at Rugby School in Britain during a football match, when a student, William Webb Ellis, picked up the ball and ran with it.
As well as carrying it, you can kick or pass the ball, but you can never pass the ball forwards.
There are two main types of rugby - union, with 15 players per team, and league, with 13. Sevens teams use only seven players, but play on the same-sized pitch.
Oval has been the compulsory shape for rugby balls since 1892. They were oncemade from pigs bladders covered in leather. The shape makes it easier to catch and hold when running.
Don't say: "Oi, mate, why are you carrying an Easter egg?"
Do say: "Forward pass! Come on, ref, he was totally offside!"
Lucky number seven
Sevens was first played in Scotland in 1883 when a rugby club needed to host a tournament to raise funds, but didn't think they could get the full quota of players. The first international competition was held in 1973, and the Hong Kong tournament began three years later.
Besides the seven-man teams, each half of play lasts seven minutes, with a one minute half-time. (But the final has two, 10-minute halves.)
Rugby v American football
Oval ball, check. Broad-shouldered men launching themselves at each other's legs and throwing them to the ground, check. Points scored by carrying said oval ball to the opposite end of the pitch, check.
The Yanks' game started as a variation on rugby, and developed into its own, hugely popular sport. Apart from differences in terminology, scoring, timing and half-time entertainment, the biggest difference is American football's use of pads and helmets and bare heads (and cauliflower ears) in rugby.
Don't say (to an American footballer): "American football is just rugby for wimps." Not if you value your bones the way they are.
Do say: "Fun facts time. Did you know, the average distance covered by a rugby player in a union match is about 20 times further than a player in an American football match? But American footballers can bench-press nearly 1.5 times the weight rugby players can - 317kg on average! Bottom line: don't get on the wrong side of them."
Players must ground the ball over the line at the opposite end of the field for a "try" worth five points. They then try to kick the ball over the crossbar and between two posts, from a spot in line with where the try was, for a two-point conversion.
Players score a drop goal where the ball is dropped on the ground and kicked as it bounces, hopefully straight between the goalposts.
Don't say: "Try harder!"
Do say: "Japan's try reminded me of Jonah Lomu's triumph over, well, pretty much the entire England team in the 1995 World Cup semi-final."
Certain traditions that go with the Hong Kong Sevens - some great, or annoying or just plain hilarious.
- The streaker/pitch invader: Generally on the Sunday, some clever soul may decide it's a good idea to leave the stands run on the pitch. Some do it naked. Last year, the police said pitch invaders would be arrested. If that's not enough to put you off, remember the 14 players on the pitch who could squash you, probably just by looking at you.
- Costumes: Almost everyone goes in fancy dress, at least on the Saturday. If you're going with friends, try to co-ordinate your outfits, either wearing the same thing, or complementary (Harry Potter, Ron Weasley and Hermione Granger, The Smurfs, or Despicable Me minions...)
- The haka: One of the most popular (and successful) teams is New Zealand. It's not just that the All Blacks are amazing players; when they win the final, they perform the haka, a Maori challenge to the opposition. It's a sight to behold.
- Friendly rivalry: Most people back their country of origin, or the team of a neighbouring nation. If your "home" team isn't playing, root for the underdog. And at the end of the day, we're all Hongkongers; here's hoping we can repeat our success of 2010 and win the shield!
One final note
The "South Stand" is why the HK Sevens are so famous; it's the "party end" of the stadium with the boldest costumes and loudest roars. To stop underage (illegal!) drinking, under-18s are banned. If you're not old enough to legally drink, don't try to sneak in, or ask someone to buy you alcohol. It's not big and it's not clever.
You've got your whole life ahead to drink responsibly - just enjoy the event with a clear mind and conscience. Big plus: explaining to your parents why you're hungover on Monday morning isn't fun.