Here's how it works:

* Initial Velocity: When a projectile is launched, it has an initial velocity in a certain direction.

* Gravity's Pull: Gravity constantly pulls the projectile downwards, changing its vertical velocity. This downward pull causes the projectile to follow a curved path, even if it's launched horizontally.

* Horizontal Motion: The projectile's horizontal motion remains constant, assuming we ignore air resistance. This means the projectile travels forward at a steady pace while being pulled down by gravity.

Other forces that can cause a projectile to curve:

* Air Resistance (Drag): This force opposes the motion of the projectile, slowing it down and causing it to deviate from its expected path. The effect of air resistance is more pronounced for objects with larger surface areas or higher speeds.

* Magnus Effect: This effect occurs when a spinning object moves through a fluid (like air). It creates a difference in air pressure on opposite sides of the object, leading to a force that pushes the object in a direction perpendicular to its spin axis. This can cause a projectile to curve, as seen in the trajectory of a spinning baseball.

Important Note: In many cases, especially when dealing with projectiles traveling at relatively short distances, the effects of air resistance and the Magnus effect are negligible compared to gravity. Therefore, gravity is often the main force causing a projectile to curve.