Помогите понять смысл шутки на английском языке!

[economistka]

"Are your mother and farther in?" asked the visitor of the small boy who opened the door.
"They was in," said the child, "but they is out."
"They was in. They is out. Where is your grammar?"
"She's gone upstairs," said the boy, "for a nap."

[snusmumric]

grammar - grandma

[economistka]

ммм, не подумала... спасибо большое!