Eat as much as you can

A signboard outside a restaurant said:

"Eat as much as you can and let your grandchildren pay the bill".

A man entered the restaurant, ate as much as he can and when the waiter gave the bill he pointed to the signboard, "Don't you see, only my grandchild needs to pay for this bill".

The waiter said, "Sir, this is not your bill. This is your grandfather's bill".