Sitting?

Being a math person, I like to have rules or definitions to abide by as I make a conclusion. So, I wonder... how do you know when a baby has reached the milestone of sitting? Is there a certain length of time that she stays upright for before toppling? Or, should the baby pull herself into a sitting position?

Molly can sit for short times unassisted. If she is focusing at something at eye level or higher, she is pretty stable. However, she usually looks at a toy on the floor and leans over to grab it.

Enjoy these pictures: