Tongue thrust reflex prevents infants from choking on foreign objects and causes them to push food out of their mouths. Prior to 4 months, a baby's swallowing mechanism is designed to work with sucking but not chewing – between 4 and 6 months, most infants develop the ability to move food from the front of the mouth to the back instead of letting it wallow around in the mouth and get spit out.
A baby's tongue extrusion reflex begins to fade between 6 and 12 months. At this point they have the ability to prepare food in their mouth before swallowing. This is also when they will be able to get a better lip seal around a cup -- prior to this time a bottle or sippy-cup may be a cleaner option.