Ketchup is a smooth, thick, sweet and tangy sauce made mainly from tomatoes, vinegar, and sugar. It's the red condiment you squeeze onto hamburgers, dip French fries into, or spread on a hot dog.

Before refrigerators were invented, the vinegar in ketchup helped preserve it, which made it incredibly popular.

Ketchup is so common now that most people don't think twice about it, but it took decades to perfect the recipe. The thick, pourable consistency that doesn't separate into watery liquid and tomato paste required careful engineering. Some people spell it catsup, which is an older alternative spelling you might see on vintage bottles or in old books, but both words mean exactly the same thing.