fold (something) into (something)

1. To bend or crease something into a particular shape or object. He loves origami and can fold paper into all kinds of animals. What do I look like, a clown? I don't know how to fold a balloon into an animal! My dumb brother folded a piece of paper into a paper airplane and hit me in the head with it.

2. To mix an ingredient into something, as while baking. OK, the next step is to fold the flour into the mixture. At what point do we fold the egg whites into this whole thing? If I fold in the eggs, will my cake be fluffier in the end?

fold something into something

1. and fold something in to blend something, such as eggs, into batter. (See also cut something into something.) Carefully, the chef folded the eggs into the other ingredients. The chef folded in the eggs.

2. to make an object by folding something, such as paper or cloth. He folded the paper into a little bird. Wally can fold a sheet of paper into an airplane that flies.