The Datura stramonium plant contains poisonous seeds that can be used to make scopolamine. Orest Lyzhechka / Getty Images The term "truth serum" refers to a number of mind-altering drugs. These drugs make you incapable of lying, or so the theory goes. Such mind-altering drugs do exist, but their effect isn't enough to stop a person from lying. Humans have known since the time of the Roman Empire that we're more readily truthful while under the influence. That's where the idea behind the term "truth serum" comes from. Truth serum refers to a number of mind-altering drugs that are supposed...