Perhaps the best-known important accidental discovery occurred in 1922 when Sir Alexander Fleming discovered an antibiotic that killed bacteria but not white blood cells as did carbolic acid, the most common antiseptic at that time. While suffering from a cold, Fleming made a culture from some of his own nasal secretions. As he examined the culture dish filled with yellow bacteria, a tear fell from his eye into the dish. The next day, he found a clear space in the culture where the tear had fallen. His keen observation and inquisitiveness led him to the correct conclusion: the tear contained a substance that caused rapid destruction of the bacteria, but was harmless to human tissue.