Here's why:

* Gravity: When an object falls, gravity pulls it downwards. This causes it to accelerate.

* Air Resistance: As the object falls faster, it encounters more air resistance. Air resistance is a force that opposes the object's motion.

* Terminal Velocity: Eventually, the force of air resistance becomes equal to the force of gravity. At this point, the object stops accelerating and falls at a constant speed. This constant speed is called terminal velocity.

Important Note: Terminal velocity is not a fixed value for an object. It depends on factors like:

* Shape and Size: A wider, flatter object encounters more air resistance than a narrow, streamlined one.

* Mass: Heavier objects have a greater gravitational force acting on them.

* Density of the Air: Terminal velocity is lower in thinner air (like at high altitudes).