Mac has discovered that he has fingers. He stops playing with toys to check out his fingers and see what they can do. He tries moving them separately and sometimes it looks like he's saluting Mr. Spock.
Here, see for yourself:
ETA: I can't believe it took the MacDaddy more than 24 hours to remind me that "Dr. Spock" wrote the baby instruction manuals and "Mr. Spock" is the Vulcan. Thanks, MacDaddy.