Final answer:
Luck played a significant role in Alexander Fleming's discovery of penicillin when he accidentally left a petri dish open, allowing mold to grow and kill the bacteria. This mold turned out to be Penicillium, leading to the discovery of penicillin.
Step-by-step explanation:
Luck played a significant role in Alexander Fleming's discovery of penicillin. Fleming accidentally left a petri dish of Staphylococcus bacteria open, allowing an unwanted mold to grow and kill the bacteria. This mold turned out to be Penicillium, which produced the antibiotic penicillin. Fleming's curiosity and subsequent experiments led to this unexpected breakthrough.