Your baby is starting to grasp what it means when someone leaves her sight, the result of a growing understanding of object permanence. An exciting demonstration of this comes the first time she tries to imitate a goodbye wave by twisting his wrist or flapping his hand. Not all 8-month-olds can wave bye-bye, but it's a fun new feat to anticipate. It takes a certain amount of fine-motor coordination to wave in the style of the Queen (or Mom and Dad).

From babycenter.com