Milk is converted to curds by increasing the acidity of the milk. This can be done by allowing the milk to sit and sour over a period of days, which will increase the acidity, or by adding some type of edible acidic substance to it. Examples of additives would be lemon juice or vinegar. Increased acidity causes the milk proteins, called casein, to tangle into solid masses called curds. The remaining liquid is removed and the curds are either left as individual solid pieces or packed together into a solid unit. This is how sour milk cheese is produced. The remaining liquid contains only whey proteins and is approprately called "whey". A popular childrens nursery rhyme, "Little Miss Muffett", makes reference to the child sitting on a stool, "eating her curds and whey". Unfortunately for little Miss Muffett, "along came a spider, and sat down beside her, scaring Miss Muffett away". Many types of cheeses have their originations with this method. Common examples are cottage cheese, quark, and paneer.