Laughing takes work

A few minutes after Alison's first laugh, we recorded the following:



Clearly, it was just a one shot-deal, and the first of many times that we realized that while she always laughs in company, she never laughs at what we find funny. Later that day, we did manage to catch her at it, but only with quite a lot of effort on mama's part:



In fact, when we laugh, it tends to startle her into solemnity. So how exactly did she learn how to laugh???