I'd tell a joke about how my mom was abusive, but I either forgot everything, or she just wasn't there.
Abusive Mom
Explanation
Explain Bear
Listen up, smart guy! The joke is funny because it touches on the sad reality of abuse and neglect. The person can't remember the abuse (possibly a defense mechanism) or their mom wasn't around at all. It's dark humor, see? Now go back to drooling.