My 4-month-old twins both hit different milestones today - one discovered her feet, and the other learned to roll over onto her tummy...and back again.  Interestingly enough, they both know how to roll onto their sides, though they roll opposite directions - the older one always goes to the left, the younger one always goes to the right. 

We've also noticed that the older one has los lost nearly all of her hair already and has a birthmark on the back of her head, while the younger one still has some hair and tends to have less cradle cap than her 2-minute-older sister.