Dame Agatha Christie is not only the most widely read novelist in the world—her 66 novels and 14 short story collections have sold more than 2 billion copies—but she's credited with creating the modern murder mystery. In honor of her 125th birthday today, below are 15 of the countless influences the late Queen of Crime culled for her popular narratives.

1. HER IMAGINARY FRIENDS

Christie wasn't sent to boarding school like her two older siblings were, so she filled her days by inventing imaginary friends to keep her company. From "The Kittens" (with names like Clover and Blackie) to "The Girls"—other schoolchildren she pretended were her classmates (including a shy girl named Annie Gray and a frenemy named Isabella Sullivan)—Christie's wide assortment of imagined characters from childhood helped her shape the ones in her novels.

2. HER STEP-GRANDMOTHER

Christie's step-grandmother Margaret West Miller, whom she called "Auntie-Grannie," was the model for Miss Jane Marple, one of her most well-liked characters. The genteel spinster sleuth appeared in 12 of Christie's novels, and the author described her as "the sort of old lady who would have been rather like some of my step-grandmother's Ealing cronies—old ladies whom I have met in so many villages where I have gone to stay as a girl." She also attributed Miss Marple's ability to root out the guilty to her Grannie's general suspicion of others: "There was no unkindness in Miss Marple, she just did not trust people."

3. MONEY

When Christie was a young child, some family trusts fell through and her father, Frederick Miller, managed to lose or squander much of his fortune. Though still comparatively well off, her youth was marked by constant worry about the family’s financial situation, especially when her father died when she was 11. “Agatha had a fear of poverty, deriving from her memory of the sudden downward swoop of the Miller fortunes,” Laura Thompson wrote in her 2007 biography Agatha Christie: An English Mystery. “Money is central to Agatha’s writings. As both Poirot and Miss Marple [Christie’s two most famous characters] are aware, it constitutes the prime motive for crime.”

4. AND 5. THE NOVELIST GASTON LEROUX AND HER OLDER SISTER, MADGE

Christie and her sister Madge had a discussion about various detective novels they liked—“We were connoisseurs of the detective story,” she wrote in her autobiography—and the conversation turned to Leroux’s 1908 closed-door whodunit The Mystery of the Yellow Room, which is widely considered one of the best in the genre and which both of the sisters loved. When Christie mused that she’d like to try to write a detective novel herself, her sister told her she probably couldn’t create such a complicated narrative. “I should like to try,” Christie said, to which Madge replied, “Well, I bet you couldn’t.” “From that moment I was fired by the determination that I would write a detective story,” the author recalled.

6. SHERLOCK HOLMES

Though Christie came about writing her beloved detective Hercule Poirot based on the Belgian refugees she spent time with during the war, she always had Britain’s most famous sleuth in the back of her mind. “There was Sherlock Holmes, the one and only,” she wrote in her autobiography of the time she was trying to decide on what kind of detective she should create. “I should never be able to emulate him,” she said, though she did contend that her inspector needed “a grand name—one of those names that Sherlock Holmes and his family had. Who was it his brother had been? Mycroft Holmes.” Later, once she was a couple of novels in, she realized she’d absorbed more of Sir Arthur Conan Doyle’s writings than she’d intended. She was “writing in the Sherlock Holmes tradition—eccentric detective [Poirot], stooge assistant [Captain Hastings], with a Lestrade-type Scotland Yard detective, Inspector Japp—and now I added a ‘human foxhound,’ Inspector Giraud, of the French police.”

7. HER DEBUTANTE SEASON IN CAIRO

Due in part to Christie's mother's failing health and their financial situation, it was decided that Christie would have her coming-out season in the relatively cheap Cairo rather than London. "Cairo, from the point of view of a girl, was a dream of delight," Christie wrote in her autobiography. She loved her time there, and though the three months she spent at age 17 did not result in a husband, it did inspire her first attempt at a novel: Snow Upon the Desert, which went unpublished, was set in Cairo.

8. WORLD WAR I

Christie worked at a Red Cross hospital in her hometown of Torquay as a nurse during the first part of the war and eventually ended up in the hospital dispensary. In order to be licensed to hand out the drugs to physicians, she studied for the Apothecaries Hall exam and spent time learning from a chemist and pharmacist. She had nightmares about making a mistake and improperly mixing poisons into ointments, but it was while she was working in the dispensary that she that she finally decided to write a detective novel. “Since I was surrounded by poisons, perhaps it was natural that death by poisoning should be the method I selected,” she later wrote. In her collective works, Christie concocted 83 poisonings.

9. MR. P, A PHARMACIST

White studying for her apothecary exam, the pharmacist whom Christie was apprenticing under was Mr. P, “the best-known pharmacist in town.” She describes him in her autobiography as a strange man who was prone to patronizing her by calling her “little girl” and patting her on the shoulders or cheek. But one day he pulled curare out of his pocket and asked if she knew what it was. “Interesting stuff,” Mr. P told her. “Taken by the mouth, it does you no harm at all. Enter the bloodstream, it paralyzes and kills you. It’s what they use for arrow poison.” Asked why he kept it in his pocket, he replied that it made him feel powerful. “He struck me,” Christie wrote, “in spite of his cherubic appearance, as possible rather a dangerous man.” She thought of him throughout the years, and credits him for helping conceive her poisoning plotline for 1961’s The Pale Horse.

10. ARCHIE CHRISTIE, HER FIRST HUSBAND

When Archie Christie asked for a divorce after nearly 14 years of marriage, Christie was devastated. “With those words, that part of my life—my happy, successful confident life—ended,” she wrote. A few dark years followed, and a new genre. Christie wrote six romance novels under the nom de plume Mary Westmacott, and her ex “was her primary inspiration,” according to biographer Laura Thompson. And her friend, the historian A.L. Rowse, wrote that the wound left by her divorce was “so deep … it left its traces all through her work.”

11. AND 12. HER EAT PRAY LOVE-ESQUE ADVENTURE AND THE LINDBERGH KIDNAPPING

After her divorce, Christie booked a last-minute trip for herself to Baghdad. “All my life I had wanted to go on the Orient Express,” she wrote in her autobiography, noting that "trains have always been one of my favorite things." So she set out to have an adventure on her own. "I had been round the world with Archie ... Now I was going by myself. I should find out now what kind of person I was—whether I had become entirely dependent on other people as I had feared. I would have no one to consider but myself. I would see how I liked that."

Turns out she liked it quite a bit, and she happened to meet a certain archaeologist at Ur whom she would later marry. She took the journey on the Simplon line many more times in later years, including a trip during which her train was stuck for 24 hours due to heavy rains and flooding. Between that experience, and the circulating stories about a different Orient Express train that got stuck in the snow for six days, she crafted 1934's Murder on the Orient Express, one of her most popular and widely adapted mysteries. The child's kidnapping that sets the stage for the book's central murder was also pulled from the papers—she based her fictional Daisy Armstrong's disappearance on the real-life crime of the century, the 1932 kidnapping of famed aviator Charles Lindbergh's toddler.

13. MAX MALLOWAN, HER SECOND HUSBAND

In 1930, Christie remarried. Max Mallowan was a prominent British archaeologist who specialized in ancient Middle Eastern history. His work took him on digs in Iraq, Syria, and Lebanon, and Christie often accompanied him and actually helped with his work—even cleaning off ancient ivory carvings dating to 900 BCE with her face cream. Her travels with Mallowan resulted in many novels with Middle Eastern settings, like Death on the Nile and Murder in Mesopotamia, as well as an archaeologist culprit and other characters resembling their friends on a dig at Ur.

14. ACTRESS GENE TIERNEY AND A RUBELLA OUTBREAK

In her 1962 novel The Mirror Crack’d From Side to Side, Christie writes about a famous American actress who, in her first trimester of pregnancy, contracts German measles (rubella) from a fan. The baby is born severely premature and handicapped, requires a full blood transfusion at birth, and has to be institutionalized for life. Years later, at a party, a starstruck woman approaches the actress and tells her that they’d met once before, when she had broken out of her measles quarantine because she just had to meet her favorite actress. Christie took this plot-point almost verbatim from the headlines—in 1943, the glamorous Hollywood star Gene Tierney had experienced this horrific tragedy exactly.

15. TRAIN STATIONS

Christie very often wrote about locations she knew well, but once, the annoyance of a delayed train was enough to spark an idea. After her wartime novel N or M? was published in 1941, the British intelligence agency MI5 began to investigate Christie’s source material. She had named one of the characters Major Bletchley, and MI5 worried that the book’s content about German spies might be based on secondhand, classified information—one of Christie’s good friends was a codebreaker at Bletchley Park and had helped break the German Enigma cipher. Concerned, MI5 convinced her friend to find out why she’d chosen that name. "Bletchley?” she answered him. “My dear, I was stuck there on my way by train from Oxford to London and took revenge by giving the name to one of my least lovable characters."