I forgot about this but was reminded with the coming of age of our
singleton son. He is 7 months and the twins are 2 1/2 years old. The
developmental milestones are amazing to behold. The interesting
thing to remember is that twins can sometimes hit them later (and
at different times) since they tend to come into the world earlier.
 
This came from another journal I review regularly called the Parent
Review.
 
---Sometime in the seventh month, full-term babies learn that when
an object is out of sight, it isn't gone. This is known as "object
permanence," and signals other leaps in understanding that are
taking place. Try this game with your baby: hide a toy or object
she is interested in under a cloth. Pause, then pull off the cloth
to reveal the toy! After a few times (or perhaps the very first
time), your baby may wish to peek under the cloth or pull it off
herself. A simple game, and yet thrilling for your baby to play and
for you to watch. Now your baby can remember, desire, and hunt for
toys that may be out of sight. She can act on her thoughts as well
as her sense of sight. That is an enormous milestone.---
 
These leaps are amazing and I am glad that some very smart person
came up with titles that can help to cement them in my mind. The
same permanence extends to "person"in the game of peek-a-boo. Your
twins now know that you are still there behind your hands. Why else
would they smile so warmly and genuinely.