Chemistry wordplay at its finest! Formaldehyde is that pungent chemical used to preserve specimens (and nightmares from biology class). The joke brilliantly plays on "formal" vs "casual" and sneaks in a Jekyll/Hyde reference because, let's face it, chemicals have personalities too. Next time you're at a chemistry party (yes, they exist and they're exactly as wild as you imagine), drop this gem and watch as everyone simultaneously groans and updates their periodic table of puns.