"Little Johnny has a question for the teacher" joke

Little Johnny is the brightest kid in his class, and finishes way ahead of all the other kids during a maths test.
To stop him from disturbing any of the other children the teacher says "Johnny, you are so clever that I'm going to ask you an extra question. There are five birds are on a wall. You're armed with a shotgun and you shoot one of them. How many are left?"
"None." says Johnny.
"What do you mean, none?" says the teacher.
"Well, one falls dead, and the others fly away because of the noise." explains Johnny.
"Ahhh, well done Johnny. I would have said four, but I like the way you're thinking." answers the teacher.
Twenty minutes later, Johnny raises his hand.
"Miss! Miss!"
"Yes, Johnny?"
"Now, can I ask you a question?"
"Please do."
"Miss, thee girls are standing next to an icecream van, and they've all got icecreams. One is licking it, the one is biting is, and one is sucking it. Which one is married?!"
The teacher looks a bit embarrassed and says:
"Err, hm, I dont know Johnny. The one who, er...is sucking it?"
"No, Miss!" says Johnny "The one with a ring on her finger, but I like the way you're thinking!"