went and found the baby books to make sure I was accurate since my kids are older.
Under the section in Tara's that says what makes your baby giggle or laugh I wrote
6 mos - other people laughing, seeing mommy
7 mos - being turned upside down, being tickled under arms and on ribs, funny voices
9 mos - knocking down towers of blocks, falling on bottom in tub (she would stand up on the side, then let go and splash and just laugh so hard, man i had forgotten about that till I read that, was that really that early?)
12 mos - almost everything
for Elise I couldn't find that same baby book, I sure looked cause it was a great one, but off of her calendar I hung by the changing table I have
8 wks - first giggle
on the 4th month page it says she has happy smiles when asked what makes her laugh, man you really don't keep as good records for the second
at 4 mos one week old I have written down "LAUGHED!!"
so there you go