Agi/Act/Ig

do, act, set in motion

The car ignition

Turning the key starts, or ignites, the car.

Agitate

When you scratch a wound you, agitate it causing it to itch more.

Active

When you exercise you're up and moving, so you're being active.

Reaction

In chemistry, when you mix two substances together they react causing a new substance to be formed.

Pull the lever Kronk!

Pulling the lever activates the machine.
Wrong Lever, Kronk!