Things that make Keaton laugh.
Looking at him. Not looking at him. Putting his shirt on. Tickling him (obvi.) His brothers. Laying alone on his play mat. Talking to him. Being in his general vicinity.
And if he laughs too much? Hiccups. Which he doesn't mind, but we feel terrible about. We have to be careful about making eye contact with him for too long, because it almost always leads to hiccups.