"Show and Tell..." joke

The teacher asked the students to bring one electrical appliance for
"Show & Tell," and the next day every kid had something.
The teacher asks Wendy: What did you bring?
"I brought a Walkman."
"And what is it for?"
"You can listen to music with it!"
"That's nice Wendy. What did you bring Kenny?"
"I brought a 'lectrical can opener, it opens cans!"
"Well done, Kenny.
Umm, Johnny, I see you didn't bring anything!"
"Yes, I did. It's in the hall."
So the entire class goes into the hallway.
"Umm, Johnny, what is that?"
"It's a heart/lung machine hospitals use to keep your heart going."
"Whoa. What did your father say about you bringing this?"
"He said, 'AAAARRRGGGH!!!'"