Here's why:

* Gravity: As an object falls, gravity pulls it downwards, causing it to accelerate.

* Air Resistance: As the object speeds up, it encounters increasing air resistance, which acts in the opposite direction of gravity.

* Equilibrium: At a certain speed, the force of air resistance becomes equal and opposite to the force of gravity. At this point, the net force on the object is zero, and it stops accelerating.

* Constant Speed: Since the forces are balanced, the object continues to fall at a constant speed, known as terminal velocity.

Key Points:

* Terminal velocity depends on the object's shape, mass, and the density of the air.

* A skydiver, for example, can reach terminal velocities of around 120 mph (190 km/h).

* Objects with a larger surface area or less mass will have lower terminal velocities.