It shouldn't surprise anyone that 18th-century lothario Giacomo Casanova had a few thoughts about birth control; he may have been the person who first discovered the contraceptive qualities of lemons.
Lemon juice contains citric acid, known today as a sperm-killer. In modern-day studies, lemon juice has been found to immobilize sperm in less than 60 seconds, making it an effective topical spermicide (or douche). Most sperm will take a day or two to reach and fertilize a waiting egg, though some may reach a waiting egg in as fast as 30 minutes, so it's not 100 percent effective. The lemon rind, too, was used as a barrier birth control method; insert half a lemon rind (squeeze out the juice and remove the pulp first) into the vagina and you've got yourself an ancient cervical cap.