A weather vane is a rotating device, usually mounted on top of a building, that shows which direction the wind is blowing. The classic design features an arrow with a large decorative shape (often a rooster, horse, or ship) on one end and a pointer on the other. The wind pushes against the larger end, causing the arrow to spin and point in the direction the wind is coming from. If the arrow points north, the wind is blowing from the north toward the south.

For centuries, weather vanes helped farmers, sailors, and ordinary people predict weather changes, since wind direction often signals approaching storms or clear skies. Sailors especially relied on them: favorable winds meant safe voyages, while sudden wind shifts could mean danger.

The term weather vane can also describe a person who constantly changes their opinions based on what's popular or convenient, spinning around like the device itself. If your friend loves soccer one week, then basketball the next, then suddenly hates all sports when the popular kids do, you might think of them as a weather vane, always shifting with the wind of popular opinion rather than having steady convictions.