What motivated Agatha Christie to write her first novel?

Her sister bet her she couldn't do it! Challenge accepted! Her first novel, “The Mysterious Affair at Styles,” was published in 1916. Christie's first published work was a poem she wrote while sick in bed at the age of eleven. Her mother suggested she write down some of the stories that she liked to talk about. She was 26 when she published her first novel. Fortunately for us, her family saw her talent and encouraged her to share it.

https://www.pbs.org/newshour/arts/agatha-christie-even-awesome-thought