It's because if you break one in half it makes a snap.

[Picking up one of the cookies, he carefully breaks the hard cookie in half until it snaps. Taking one half, he starts to eat it while offering the other to Jingyi.]

They've very good with tea. Now, come over here. You can help me by stirring this bowl for me with the wet ingredients while I get the dry ones ready to add.

[He indicates a bowl with butter and eggs in it.]