What's the difference between light and hard?
It's easy to get to sleep with a light on.
Light vs Hard
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Explanation
Experiment
This is a play on words. 'Light' and 'hard' are used in the context of sleeping, but the punchline uses 'light' to refer to illumination instead of weight or difficulty. It's unexpected and a bit silly, because when someone is having 'a hard time' sleeping, they don't usually think of a literal light.