Why is it that when someone hears this for the first time and they go look in the dictionary, they've "fallen for the joke"? "Gullible" means you believe anything you hear. So, "falling for the joke" should be if you respond with "oh, I didn't know that", then proceed to repeat this to everyone unchecked. Checking the dictionary means, "I don't believe your and I'm going to prove it."
At least, this is how I see it. *shrug*