Here’s Nina Roth, the skip of the U.S. women’s curling team at practice. Curling is a centuries old sport in which the players deliver really heavy granite rocks on a long sheet of ice toward a target. There are four players on each team. On each shot, one player throws the rock, two are designated sweepers, and one sets the target. The curling rocks themselves weigh about 42 pounds a piece. The type that’s used at the Olympics has made of dense granite hailing from Scotland. Because of friction with the ice, a rock will naturally curl as it travels down the sheet. So to control that curl, players give the handle a little turn, either clockwise or counterclockwise, just before they deliver the rock. They release the handle before reaching what’s called the hog line. Sweepers get out in front of the curling rock to heat up the ice and reduce friction, which allows the rock to travel farther down the ice. They can also alter the path of the rock with a tactic known as directional sweeping. With wins and losses often decided by centimeters, the importance of sweeping is impossible to overstate. The target is on the other end of the sheet. Known as the house, it’s a series of concentric circles that forms a sort of bull’s eye. Each game takes place over the course of 10 ends. The twist is that only one team can score in each end, and that point goes to whichever team has a rock closest to the button, or the center of the target. If that team has two rocks closest to the button, they get two points. And so on. A total of 16 rocks, eight per team, are thrown during each end. Simple right? Let’s take a look at that whole sequence again.