Why does Triton always keep the same face toward Neptune?

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Clouds in Neptune's atmosphere — Neptune A photograph of clouds in Neptune's atmosphere, taken by Voyager 2 in August 1989. The view is from below the planet's equator, and north is up. The Great Dark Spot (center left) is 13,000 km (8,100 miles)—about the diameter of Earth—in its longer dimension. Accompanying it are bright, wispy clouds thought to comprise methane ice crystals. At higher southern latitudes lies a smaller, eye-shaped dark spot with a light core (bottom left). Just above that spot is a bright cloud dubbed Scooter. Each of these cloud features was seen to travel eastward but at a different rate, the Great Dark Spot moving the slowest.

Triton always keeps the same face toward Neptune due to a phenomenon known as synchronous rotation.1

This occurs because Triton rotates once on its axis in exactly the same amount of time that it takes to complete one revolution around Neptune—a period of 5.877 days. As a result of this orbital locking, the moon maintains the same orientation relative to the planet at all times.1