My 3 month old baby laughed for the first time today! It was such a sweet and joyous sound

I love first laughs because they are usually unexpected. Today my baby laughed because her big sis (my 3 year old) and I were cooing and tickling her on the couch in our living room... DH witnessed it too which was nice because he was at work when our eldest first laughed.

When did your baby first laugh? What did you do to make him/her finally laugh?