String cheese is a type of cheese where the manufacturing process causes the cheese to become stringy. Mozzarella cheese commonly becomes stringy when heated. In the United States, string cheese is used to refer to cylindrical pieces of cheese, called "cheese sticks," usually either mozzarella or a combination of that cheese and cheddar. String cheese can be eaten by pulling "strings" of the cheese off the cylinder, and it is commonly eaten as a snack food.
This day was apparently founded by Galbani Cheese back in 2017.
Image: Chris Hsia from United States, CC BY-SA 2.0 <https://creativecommons.org/licenses/by-sa/2.0>, via Wikimedia Commons
