"There is a substance [caffeine] in tea and coffee which when taken into the human system, tends to increase the beating of the heart; which in turn increases the rapidity of the circulation of the blood and of breathing. This causes the body to become warmer and more exhilarated. After a time, however, this temporary enlivenment passes off, and the body is really in a greater need of rest and recuperation than it was before the beverage was taken. Stimulants are to the body what the lash is to the lagging horse—it causes a spurt forward but gives no permanent strength or natural nourishment. Frequent repetitions of the lash only make the horse more lazy; and the habitual use of strong drink, tobacco, tea, and coffee, only tends to make the body weaker and more dependent upon the stimulants to which it is addicted."
-David O. McKay
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Words to live by, as far as I'm concerned.