A curveball is a type of pitch in baseball that spins as it flies through the air, causing it to curve or break sharply just as it reaches the batter. Instead of traveling in a straight line like a fastball, a curveball drops and moves sideways, making it much harder to hit. Pitchers throw curveballs by gripping the ball in a special way and snapping their wrist as they release it, creating spin that bends the ball's path through the air.

The word also means an unexpected challenge or surprise that's hard to deal with. When your teacher suddenly announces a pop quiz on a chapter you haven't reviewed yet, that's a curveball. When you're planning a birthday party outdoors and it starts raining, the weather has thrown a curveball.

Just like a batter facing a curveball has to adjust quickly, someone dealing with an unexpected problem has to think fast and adapt. You might hear someone say “life threw me a curveball” when something surprising disrupted their plans.