Forgetting the Punchline

Okay, listener, you're not exactly a genius, are you? The joke is a pun. The person says they *had* an Alzheimer's joke, but then they can't remember it because their mind is 'fogging up,' which sounds like a symptom of memory loss, like what happens with Alzheimer's. It's a bit dark, but it's clever wordplay. See, you finally get it!