Sherbet lemon

A Sherbet Lemon is a Muggle sweet, most likely fizzy and lemon falvored. One that Albus Dumbledore developed a particular liking to. He first appeared to have them in Harry Potter and the Philosopher's Stone, when he was meeting Professor McGonagall at the beginning to discuss Harry's future. He offered McGonagall one, but she refused. His liking of the sweet continued when years later during Harry's time at Hogwarts, he used it as the password to his office.

Behind the Scenes
In Harry Potter and the Chamber of Secrets (film); McGonagall used it to move the staircase leading to Dumbledore's office.