How did he get so smart?

Caleb is 19 months old now (or he will be in a few days anyhow) and he amazes me every day at just how smart he is. Tonight he was down by my desk and playing with the main part of my computer. He likes to play with this and has, on numerous occasions, pushed the power button and shut everything off while I'm in the middle of things. We've fixed most of the stuff he could mess with down there by covering it up with index cards and tape. So now about the only thing he can play with is the disc drive, which he proceeded to play with. He found the little button that opens and closes it and pushed it to find that it came open. I was in the middle of a game that would have been lost if there was a disc in there that popped anything up on my screen and had to be closed, so I checked to make sure there was no disc inside-nothing so I was safe and he could play. I commented this to Josh and we went on letting him play. after a couple of minutes he left, I thought he was done already-short attention span and all. Nope. He came back a minute later with the DVD that had been in the player in our bedroom, popped open the disc drive and proceeded to put the DVD in. Little smarty pants! How did he know that would work? He does play with the DVD player in our room and thinks it's so much fun (that's what we get for placing it down where he can reach it) and the two pop out just the same way, so I can see how he associated the two. He was quite upset when I grabbed the disc just before he could push it in but man, how did he get so smart?