What is Biathlon? It’s cross-country skiing with guns.

A decathlon has 10 events, a heptathlon has seven and a triathlon has three. Biathlon therefore has only two. And a strange couple: cross-country skiing and shooting.

This quirky competition has been contested in various forms since the 18th century and has been part of the Olympics since 1924 (when it was known, more accurately, as the Military Ski Patrol).

The modern biathlon is a standard cross-country race with periodic interruptions in which athletes pull a .22 caliber rifle from their backs, point it at a target, and fire. Missed targets cost time and sometimes require racers to ski a 150 meter penalty lap. (Minimum rifle weight is 3.5 kilograms, or about eight pounds.)

You might be wondering how adding shooting to a cross-country race can add excitement. Yet it is.

In a typical cross-country race, the best skiers pass quickly. There may be a late pass or two, but medalists can usually be identified early.

But in biathlon, the shooting aspect is a wild card that casts doubt on every race until almost the end. A leader who misses a shot or three can fall back wide due to penalty rounds. A skier who appears to be behind can hit all targets and jump forward.

One of the sport’s incongruities is the drastic difference in the physical skill sets that the two disciplines require. Cross-country skiing is a challenging aerobic sport, rewarding exceptionally fit athletes. Shooting requires steadfast calm and precision, to the point that world-class athletes sometimes try to time their shots so they get caught between heartbeats.

Five-time gold medalist Martin Fourcade has retired, but another Frenchman, Quentin Fillon Maillet, has put on his skis as the best male biathlete. France, the Scandinavian countries, Germany and Russia are expected to account for the bulk of the top rankings.

If you don’t know anything about a winter sport, it’s usually a good guess that Norway is good at it. And indeed, Norway’s Marte Olsbu Roiseland leads the women’s Biathlon World Cup circuit. But the Swedish sisters Elvira Oberg and Hanna Oberg will challenge her in all tests.

The Americans have not won any medals on the World Cup circuit this year and have never won a medal in biathlon at the Games.

Events at the Olympics include the individual, a time trial of approximately 10 miles; the sprint, which is about half as long; pursuit, in which skiers start according to their sprint finishes; mass start, in which everyone starts at the same time; and men’s, women’s and mixed relays.