My friend Katie's mom, who has five grown kids, believes that all fussy babies grow up to be really smart, and they're fussy because they're bored.
Gracie definitely fusses when she's bored. Also, although there was never a switch-flipping moment, like people talk about with classic colic, it was right around the four month mark, when she could actually play, that Gracie got a lot less fussy. The world just didn't seem to frustrate her as much when she could grab her doll, shake her rattle, bring things to her mouth, etc. Even before I heard Katie's mom's theory, I would joke that Gracie was fussy because she's brilliant and tortured by her thoughts.
Tonight, as I was rocking her and trying (unsuccessfully, natch) to get her drowsy enough for bed, I watched her playing with a burpie. She shook it around, held it up, chewed on it, shook it again. I thought, is that it for you? Are you just too smart for your own good?
Right as I thought that, she stuffed the burpie into her mouth, bit down, and proceeded to get supremely pissed off when she couldn't get it back out of her mouth.
...well. At least she's cute.