How'd he get so smart so fast??

Nathan's almost 5 months old. It amazes me continually how much he's grown and how incredibly smart he already is.
Over the weekend Nathan's mobile came on spontaneously during the night. Actually, it was a result of him rolling over and kicking it. I think the first night it was a pure coincidence. Now, almost a week later, I think the kid knows what he's doing.
You see, we use the mobile as a means to get him back to sleep at night. He's doing pretty good about sleeping, but occasionally wakes up. He's got this really nifty Fisher-Price mobile that has a light on top that displays star and moon shapes on the ceiling. It also plays lullabies and classical music tunes.
At 10 til 5 this morning I woke up not hearing Nathan, but hearing "Twinkle, Twinkle Little Star." A few seconds after that registered, I heard him making noise and sucking his fingers.
Bless his little heart ... the mobile does work to get him back to sleep. Unfortunately, so does sucking. Now if he could just figure out how to get the pacifier back into his mouth .... but I'm sure that's coming any day now, too.