It started with Davis promising to be careful while drinking from his cup without a lid on the couch (Daddy's idea). He has just started with the cup, so each time has the potential for spectacular spills. If he gets distracted by a little drip, he looks down (and angles the cup down) and the entire contents get spilled-out. It seems to surprise him each time it happens. With some positive re-enforcement after every successful sip, Davis should be drinking with the big boys any day!
Maybe Davis thinks it's funny that we get nervous whenever he has an open container. Whatever it is, he enjoys going from the all-serious look to the super-cheesy smiles!