16 year old girl goes to store and picks a pack of Budlight

She then goes to cashier to pay for it. Cashier tells her it costs $15.99. She pulls out $20 and gives it cashier. Cashier takes it and give her a change. The girl very surprised asks:

The girl - "why are you not asking for my ID??"

cashier - "we don't ask for IDs if someones wants to buy water"