Interesting fact: (oh, the things you learn from
Behind the Music!)
When "Sandy" is being sung by John Travolta in Grease, he's sitting on a swing in front of the drive-in screen. He's just been rejected by Sandy cause he tried to... ahem... get it on with her. There is a trailer that's playing behind him. It's got cheesy dancing products and such.
Anyway, the producer had tried to fill the movie with as much sexual tension as was allowed by the Hollywood censors. He had come across this trailer and chose it for it's fantastic sexual innuendo. The trailer has been purposefully timed so that just as Danny sighs "Oh Sandy" at the end of the song, a hot dog leaps into a bun behind him.
And I always thought Grease was so innocent...