Arlo has an amazingly broad range of facial expressions. Here are a few.
(Click on any photo for full resolution version.)

Fleeting fast changes

Complaining
Complaining a little.	Complaining a lot.
Grumpy (his signature expression)	My hands are dirty!

Tongue out

Happy
Happy!	What's over there?
Happy	Whee! Watch that robot go!

Focused
Are you sure that robot is safe?	Look at that sand fall.	Interested