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Sweeping briefly melts the frozen droplets, called pebble, that cover a curling sheet, creating a microscopic film of water that reduces friction under the stone. Two things follow. The stone travels further, by several metres if it is swept hard for its whole journey, and it curls less, holding a straighter line. Sweepers are therefore correcting two errors at once: a shot thrown a little light can be carried to the target, and a shot drifting too wide can be kept straight. The skip watches from the house and calls when to sweep; the sweepers judge speed themselves, usually against a stopwatch time taken between the hog lines.

The hammer is the last stone of an end, and having it is worth roughly a point at elite level because you get to react to everything the opposition has already done. It is decided before the game by the last stone draw, where each team throws to the button and the closer average wins, and after that it goes to whichever team did not score in the previous end. That rule is the strategic heart of curling: scoring a single point with the hammer is often a bad outcome, because you hand last stone back for only one point. Teams would rather blank the end, keep the hammer, and try again for two.

Because it is released with a slow rotation, two or three turns over its whole 30-metre journey, and that rotation makes it drift sideways by about a metre. The strange part is the direction: a clockwise-rotating stone curls to the right, which is the opposite of what a spinning ball does on a table. Physicists have argued about the mechanism for decades, with competing explanations involving asymmetric friction at the front and back of the contact ring, and tiny scratches the stone leaves on the pebble that later stones follow. There is still no single accepted theory, which makes curling one of the few sports whose central phenomenon remains genuinely unexplained.

The delivery itself demands balance, flexibility and repeatable precision rather than power, but sweeping is straightforwardly hard physical work. A sweeper covers a couple of kilometres over a game while pushing down hard on a brush, in short bursts that drive the heart rate close to maximum. Elite teams train for that specifically, with gym programmes built around core stability, single-leg balance and upper-body endurance. The mental load is comparable to golf: every shot is planned, discussed and executed under a running clock, and a world championship is nine days of two-hour games where a centimetre decides the result.

Mixed doubles is a separate Olympic discipline played by two people, one of each gender, over eight ends instead of ten. Each team delivers five stones rather than eight, and one stone of each colour is placed on the ice before every end, one in the house and one as a guard, so six stones per team are in play. One player throws the first and last stone of the end, the other the middle three, and both sweep. Once per game a team may call a power play, shifting the pre-placed stones to one side of the sheet to open up a bigger score. It is faster, higher scoring and far more volatile than the four-player game.

Nearly all competition stones are cut from one of two granites: Blue Hone and Common Green from Ailsa Craig, an uninhabited islet in the Firth of Clyde, or a granite from the Trefor quarry in north Wales. Ailsa Craig rock is prized because it absorbs almost no water, so it does not crack when repeatedly frozen and thawed. The island is a bird sanctuary, so quarrying happens rarely and in bulk, with a 2,000-tonne harvest in 2013 and another in 2020, and Kays of Scotland has held exclusive rights for well over a century. A matched set of sixteen championship stones costs several thousand pounds but lasts for decades.