My favorite joke ever is one that my nephew made up when he was 4 and just starting to understand the patterns people use to make jokes. That nephew is in high school now, and I still use how a person reacts to the joke as a measuring stick for how well I'm likely to get along with them. Without further ado...Q: What has three legs, but isn't spaghetti?A: A spider.
The lesson? Never steal a punchline if someone else is telling a joke. Saying, "Oh yeah, I love this joke" is fine after the joke's initial setup, but never steal the punchline from the storyteller. Huge no-no.