Riddle me this...

As we drove home from the airport Saturday, my daughter was sipping tamarind soda from a plastic cup. She exclaimed a couple times that she got a tiny little shock on her lip when the soda touched it. This led to passing the cup around the car and everyone having the same phenomenon. We wondered how soda from a plastic bottle could be poured into a plastic cup and generate a static charge. Any science type folks wanna do some 'splaining?