The music for this song was written by Charlie Chaplin for the score of a 1936 movie he directed called "Modern Times". With lyrics by John Turner and Geoffrey Parsons in 1954, it became a popular standard. It was reportedly Michael Jackson's favorite song, and his version of the song was almost released as a single in 1997.
Smile, though your heart is aching
Smile, even though it's breaking
When there are clouds in the sky