The key to the poisoner's art lies in a thorough understanding of how a substance's effects on a person vary depending on amount or concentration of the substance. Let us take for an example leaves of Heart's-Ease, a common anodyne found in every pharmacopeia. The leaves are brewed as a bedtime tea by those who have trouble falling asleep. However, if the leaves are dried and converted to a powder, one can create a powerful soporific. Six grains of this, added to a night-watchman's ale, are almost guaranteed to put him in a deep sleep within a quarter-hour. Give him twenty-four grains, and he'll never wake again.