"Farmyard Noises" joke

Teacher enters the classroom and asks the grade two students to learn some farm yard animal noises so they could perform them at the school assembly.
The day comes quickly and it was Grade Two's turn to impress the rest of the school. The teacher introduces them
"Mary, can you do a farmyard noise?"
Proud as punch Mary says, "Yes miss... mooooooo!"
"Michael, can you do a farmyard noise?"
Grinning ear to ear he replies, "Yes miss... oink oink oink!"
"Sally, can you do a farmyard noise?" Jumping on the spot she says, "Yes miss... cluck, cluck!"
"Johnny, can you do a farmyard noise?"
Johnny was quiet for a while then says "Yep"
"Well can you show us?" Prods the teacher
"Ok then... Johnny get of the ****ing tractor you little ****!"