The history of medicine.
"I have a sore throat."
2000 BC : "Eat this root"
1200 AD : "That root is heathen, say this prayer."
1500 AD : "That prayer is superstition, drink this elixir."
1800 AD : "That elixir is snake oil, Take this pill."
1900 AD : "That pill is ineffective, Take this antibiotic."
2000 AD : "That antibiotic is artificial. Here why don't you eat this root."
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A linguistics professor was lecturing his class.
"In English," he explained, "a double negative forms a positive. In some languages, such as Russian, a double negative is still a negative."
"However," the professor continued, "there is no language wherein a double positive can form a negative."
Immediately, a voice from the back of the room piped up: "Yeah..... right...."