Mayonnaise bottles (full ones)! About half of the main Gintama characters have ridiculous food obsessions, and Hijikata's is the worst. He puts copious amounts of mayonnaise on/in everything, even sweets and drinks, guzzles it straight from the bottle, and doesn't understand how everyone else doesn't like it just as much. In hindsight it's a wonder Noiz never learned that about him on Prismatica since he won't shut up about it either!