Final answer:
Caffeine is the natural stimulant added to some beverages, which works by blocking the neurotransmitter adenosine, thereby increasing alertness and arousal.
Step-by-step explanation:
The natural stimulant that might be added to some beverages is caffeine. Caffeine is a central nervous system (CNS) stimulant belonging to the methylxanthine class and can be found in coffee, tea, guarana, and other plants. It is known for its ability to increase alertness and arousal. The mechanism of action for caffeine involves blocking adenosine, a neurotransmitter that promotes sleepiness, and thus preventing drowsiness. With an effective dose of about 200 mg, which equates to roughly two cups of strong coffee or tea, caffeine is the world's most widely consumed psychoactive substance, and it is legal and unregulated in nearly all parts of the world.