A painting of a fruitbowl was the entrance to the the Hogwarts kitchens. If one tickled the pear, it laughed and turned into a door knob. Fred and George were one of the few who knew how to enter. It is possible that they did so with the help of the Marauder's Map. In the 1994–1995 school year, they passed this information to Hermione Granger who entered the kitchens and met Dobby for the first time since he began working there.