Alia And The Coffee Riddle

Aalia had returned her cup of tea because a fly was in the cup. She got angry when she realized the waitress had just given her back her old cup of tea thinking she wouldn't know. How did she know it wasn't changed?

The tea was sweet because she added sugar; if it was a new cup, it would have been bitter. The tea was sweet because she added sugar; if it was a new cup, it would have been bitter.