How do our children grow from little girls to young women?

How do you know when you children are growing up?  You know when you go to a wedding and they are no longer dancing with Dad, uncles, and grandfathers.  You know when you can see boys checking her out and one obviously crushed when she danced with another.  When did this happen?  When did she go from a cute little girl dancing on her daddy's feet to a young woman.  I swear I only blinked and she is all grown up.