Someone's ex is getting compared to phenolphthalein, the ultimate mood-swing molecule of chemistry! This compound is famous as a pH indicator that dramatically shifts from colorless in acidic solutions to bright pink/purple in basic ones. The joke works on two perfect levels: First, the chemical actually does change color faster than you can blink during a titration. Second, it's drawing that classic parallel to dating someone whose emotional state flips just as rapidly and unpredictably. Next time your chemistry professor demonstrates a titration, just whisper "reminds me of my dating history" and watch them either laugh or back away slowly.