Each of our boys has had a unique habit developed at a few months that seems to stick with them. For Jackson, it was stroking and pulling his hair. It started whenever he would be taking a bottle as a baby. Today he still does it if he is tired, bored, or nervous.
Bryce so far seems to be developing a habit of covering up his face. Anything we give him in the crib or chair gets pulled onto and over his face. If you want to play peek-a-boo, he smiles and laughs when you cover his face up, not when you reveal yourself to him. See this video for an example of how he loves to cover himself up. At first he seems like he is getting smothered but when you uncover him he just wants to pull it back over his face.