MATH PUZZLE: Can you replace...

One day in class, the teacher brought a bag full of fruit and said, "Now class, I'm going to reach into the bag and describe a piece of fruit and you tell me which fruit I'm talking about. Alright, the first one is round, plump, and red. Little Johnny raised his hand high but the teacher ignored him and picked Deborah who promptly answered, "Apple." The teacher replied, "No Deborah, it's a beet, but I like your thinking. Now the second one is soft, fuzzy and colored red and brown." Johnny is hopping up and down in his seat trying to get the teacher to call on him but she calls on Billy. "Is it a peach?" Billy asks. "No, it's a potato, but I like your thinking," the teacher replies. "Okay the next one is long, yellow, and fairly hard." Johnny is about to explode as he waves his hand frantically but the teacher calls on Sally who say, "A banana." The teacher responds, "No, it's a squash, but I like your thinking." Johnny is irritated now so he speaks up loudly, "Hey, I've got one for you teacher. Let me put my hand in my pocket. Okay, I've got it. It's round, hard, and it's got a head on it." "Johnny!" she cries, "That's disgusting!" "Nope," answers Johnny, "It's a quarter, but I like your thinking!"