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.