From Rolling Stone:
Sonny and Cher were living in their manager’s house, where Bono would go down to the garage and bang out songs all night on an upright piano. One night, he woke up Cher and asked her to listen to his latest tune, “I Got You Babe,” and sing the lyrics, which he had written on a piece of shirt cardboard. She thought it was OK but really wanted a song that modulated. So Bono changed the key at the bridge and woke Cher up once again hours later to hear it; she was delighted. Sonny and Cher had their first Top Forty hit, and their only Number One record.