Final answer:
Caffeine is a naturally occurring substance that is classified as a stimulant, whereas nicotine is another naturally occurring substance that is also classified as a stimulant.
Step-by-step explanation:
Stimulants are substances that stimulate the brain and increase alertness and wakefulness.
They are psychoactive drugs that have an activating effect on the central nervous system. Not all naturally occurring substances are classified as stimulants, however.
For example, caffeine is a naturally occurring substance found in coffee, tea, and some soft drinks that acts as a stimulant.
On the other hand, nicotine is also a naturally occurring substance but it is classified as a stimulant. Other examples of stimulants include cocaine and amphetamines.