Zipper

Zipper is a natural sign. Take both hands and pinch the thumb to the index finger on either hand. Your non-dominant hand stays put at the imagined location of the zipper, while the dominant hand makes the motion of unzipping and zipping from the "base" where the non-dominant hand is positioned.

Make the sign for zipper wherever you want your baby to mimic the action, on his or her clothing or backpack, for example.