Grandma's house

Little Johnny and his family were having Sunday dinner at his Grandmother's house. Everyone was seated around the table as the food was being served.
When little Johnny received his plate he started eating right away.
"Johnny wait until we say our prayer."
"I don't have to." The boy replied.
"Of course, you do," his mother insisted.
"We say a prayer before eating at our house."
"That's our house," Johnny explained. "But this is Grandma's house, and she knows how to cook!.