Early communication skills are important for young babies and children. They are born with an inbuilt desire to connect to others and part of this is their ability to mimic facial expressions. Babies as young as 6 weeks have shown the ability to copy the expressions that their parents have made to them.
My favourite is sticking my tongue out and watching them copy back to me. I have always done it and never really known why until I started to learn about child development in the 90's. I guess it is hardwired in to us adults as well!
Other fun things to try include making your eyes wide or opening and shutting your mouth. Very often the baby or toddler will make the same face back to you.
This isn't just play. It is early language and communication skills. Babies are primed for social interaction and have a preference for zoning in on faces. They respond to you when you look at them and make eye contact, but if you look away they will go quiet and look somewhere else. They can pick up on your expression and many tests have shown that they respond better to someone that smiles rather than a neutral facial expression.
This early form of non verbal communication is all part of the important development for successful language and social skills. This sounds like a perfect excuse to sit and play with baby for as long as possible!