"You can't catch a fish when your fishing rod is missing a hook." The same principle applies when crafting a song.
The hook may be the title of the song, a lyrical line (usually repeated) that summarizes what the song is about, a rhythmic passage or an instrumental part called "riff." It is mostly evident in pop music; songs that have made it to the top of the charts have unforgettable hooks. Here are some examples:
"Don't you forget about me
Don't, don't don't don't
Don't you forget about me"
"With or without you
With or without you, oh
I can't live with or without you"
"Nothing compares
Nothing compares to you"
"And I will always love you
I will always love you"
"You're beautiful, you're beautiful
You're beautiful it's true"
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