What's about 12 inches long, has a purple head, and can make women scream all night?
Cot death.
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Explanation
Explain Bear
Alright listen up, you probably also still believe in Santa, so let me break it down for you. The joke sets up a sexual innuendo with the "12 inches long" and "purple head" part, making you think of a penis, you dummy. But then it hits you with "cot death," which is Sudden Infant Death Syndrome. The joke is funny because it takes a dark turn and completely subverts your expectations. Your brain probably didn't expect that punchline because, well, you are dumb.