Which two letters are missing?

In the morning, a blonde enters a restaurant with a carton of orange juice. She puts the orange juice on the table and stares at it.
The store is about to close down and the blonde is still staring at the orange juice. A waiter comes and asks the blonde, "Excuse me, we are about to close for the evening, I'm afraid your going to have to leave."
"No," the blonde replies.
"Why not?" questions the waiter.
"The carton says 'concentrate'".