If he's a fun-loving ENTP: Cock one eyebrow up and walk in sultrily with a slight smile on your face while he is working on something. When he notices you, if he glances at anything but your eyes for a second or two, or returns the smile, he's likely in a good mood. Sit on the desk near him, wait until he looks at you again, arch that eyebrow, lick your lips provocatively and then knock some of his stuff on the floor. Pull the squirt gun from behind your back and begin firing. Laugh and run like hell.
I take no responsibility for what happens to you afterwards.