Nothing lasts forever, and packets of ketchup are no exception. The condiment is so popular at fast-food chains that suppliers design the product for a quick turnover. So if you're hoarding packets in your cupboard, watch out. Heinz suggests a nine-month shelf life, but McDonald's thinks that six is long enough. One telltale sign: the older ketchup is, the darker and thicker it gets. So if you're squeezing burgundy paste onto your plate, chances are it's past its sell-by date. And who'd want fries with that?