Not knowing what else to do, Aunt Purl went on planting her tulip bulbs very carefully. Whatever happened she was determined to finish planting the garden. Suddenly she turned and saw Tink take something out of his pocket. It was a water pistol and he explained that it contained Amyl nitrate, which was a powerful heart stimulant. He began squirting her with it, knowing that it would cause a fatal heart attack and that by the time her body was found the rain that was predicted for later that day, would have washed all traces of the amyl nitrate away.
When Ms. Dunnit called to pay a visit the next day, she found Purl's corpse in the garden and called the police. The coroner ruled death by natural causes and no one (except Ms. Dunnit) saw any reason to doubt it. Tink inherited the bulk of her estate, except for the house and gardens, and promptly began squandering it on gambling and other unsavory activities.
In the spring Ms. Dunnit paid a visit to the police and told them she knew that her friend Purl had been murdered and she knew who did it!
How did Ms. Dunnit know that Tink had killed her friend?
E-mail your answers to me at firstname.lastname@example.org (please don't comment on the blog - you'll give it away!) by Saturday, May 17.