TIL that only 1 song used in the film Singin' in the Rain was entirely new. All of the other songs, including Singin' in the Rain, had already been used in previous films in one form or another.
Don't make me come over there with my "Gene Kelley had a high fever when performing the famous Singin' in the Rain scene and the rain was made visible by adding milk, making the whole place stink for days" facts. Because I'll do it.