What is the source of casein glue?

The correct source of casein glue is milk. Casein is a type of protein found in milk and is responsible for the glue's adhesive properties. It is extracted from milk as a by-product during the cheese-making process. Casein glue has been utilized for centuries due to its strong bonding capabilities and its ability to adhere well to a variety of surfaces, making it useful in woodworking and other applications.

In contrast, the other options do not provide a source for casein glue. Eggs are primarily associated with egg whites and yolks, which can be used in different adhesives or culinary applications, but not in casein glue. Corn and wheat are both sources of starch, which can produce adhesive properties but do not relate to casein glue, as they lack the protein structure that casein provides. Thus, milk is the correct answer as it directly supplies the essential protein necessary for creating casein glue.