Big Girl

When did my little girl get so big?  This morning Lizzie got up all by herself and came downstairs to join me and the twins in the kitchen.  I went upstairs to get her clothing and when I came back down I didn't see her in the kitchen but quickly heard her say "Mommy, I'm going pee".  I went in to the bathroom expecting to see her on her little pink Bjorn toilet but what I really saw was her stool pulled up to the big toilet and Lizzie sitting on the edge.  She didn't even put her toilet seat on it.  She was holding on and doing her business, all by herself!  How did this happen?