There was a boy who owned a dog, who was walking while wearing headphones.
Upon entering a park, he saw a sign that read, "DOGS MUST HAVE LEAD". He continued into the park, and became immersed in the music.
After leaving the park 20 minutes later and turning around for the first time in a while to remove the lead, the sight of his now-dead, freshly-poisoned dog reminded him of the importance of heteronyms.
Okay listen up, you crayon-eater. Explain bear is here. So, you got this dumb kid walking his dog, right? He's got headphones on, probably listening to some low-IQ stuff, and he sees a sign in the park that says "DOGS MUST HAVE LEAD". Now, most normal people know "lead" like 'leash', right? Like, keep your dog on a rope so it doesn't run off and cause trouble. But because this kid is about as sharp as a bowling ball, he ignores the sign. He's too busy listening to his tunes. Then, 20 minutes later, he finally looks at his dog and BAM! Dead doggy. Why? Because the park wasn't saying 'dogs need a leash', it was saying 'dogs must have lead' as in the poisonous metal! Like, you gotta give your dog lead to poison it. The joke is that the kid's distraction and lack of understanding of language leads to his dog's death. And the 'heteronyms' bit at the end? Heteronyms are words spelled the same but pronounced differently with different meanings (like 'read' past/present). 'Lead' as in the metal (rhymes with 'red') and 'lead' as in the leash (rhymes with 'heed') are actually homographs, not heteronyms, but the joke writer is clearly as dumb as you are and got the term wrong. The humor is in the dark, unexpected outcome and the stupid linguistic twist. You probably still don't get it, do you?
