Large balloons filled with Nitrous Oxide AKA "Laughing Gas."
The term Ice Cold Phatties comes from the unscrupulous hucksters who try to sell them to you in a concert or festival parking lot. "Ice Cold" because they're straight from the metal tank and literally cold, and "Phatty" because the buyer always wants the largest balloon they can find.
Get your Ice Cold Phatties! $15 or 3 for $40!