What is the difference between light and hard? You can go to sleep with a light on.
Light vs. Hard
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Explanation
Explain Bear
Okay, so basically, the joke is playing on the double meaning of "hard." Like, you can sleep with a light on because it's just a regular light. But you can't sleep with a boner, ya feel me? It's all about the innuendo, which you probably don't get, since you're not so bright.