Balloon fun

D. Leela and classmates, Chennai

Leela and Maya were quarreling when Nivedita came by. "What's all the noise about?" she asked.
Maya said, pointing to Leela, "She's really mean. She keeps hiding the cellotape and bursting balloons to disturb me. I'm not able to finish building my train model." 
Leela laughed, "What train. Just some card-board cut-outs that won't even move. Not like my balloon!" Saying which, she let go of a fully inflated balloon which whooshed past Maya's ear like a high-speed train.
As Maya started to yell, Nivedita said, "Come, Leela, that's not nice. Don't tease her." Then she said to Maya, "You know, you can actually make a train with the balloon. I know some other balloon tricks also." 
Nivedita took a reel of thread and tied one end to the window grill. She picked up a ball-point pen from the floor and checked the refill. "It's nearly empty, good," she said, and cut off the front end of the refill to get a short straw.
She pushed the end of the thread into the straw/refill and pulled the thread all the way through the refill by simply sucking from the other end! She then tied the other end of the string to a chair at the other end of the room, keeping the long string taut.
Leela and Maya watched in fascination as the refill bobbed about on the string. Nivedita picked up the fallen balloon and started to blow it up. Then, holding the mouth of the balloon tightly closed, she told Maya, "Cut me a piece of cellotape." She use the cellotape to stick the side of the balloon to the refill. Then she let the balloon go.
As the air rushed out of the balloon, the balloon and refill started to slide over the string, and then speeded up like a train on a track. Maya clapped her hands and shouted, "Once more!" 
Leela asked, "What makes it do that?" Nivedita said, "Look, as the air comes out of the mouth of the balloon, it kicks the balloon backwards. This is what Isaac Newton called the third law of action and reaction. So the balloon moves backwards just as fast as the air comes out. Since the refill is hanging freely from the string, it also slides on the string." 
They all spent some time playing with the balloon-train. Then Leela said, "Nivi, you said you knew some other balloon tricks too. Can you show us?" 
Just then Mother came to the children's room. "Milk time," she said. Come along, everyone. "Yeah!" shouted Maya, the milk lover. As Leela hung back, Nivedita asked, "Aunty, can you boil some water for us?" 
"Sure," said Mother, "but be careful. What do you want it for?" 
"An experiment," said Nivedita. She picked up the balloon, removed the tape from it and fixed its mouth tightly over an empty plastic water bottle. The balloon hung limply over the side. She set the bottle in a dish. "I want to put the hot water in the dish that the bottle is sitting in," she said. "Oh, I see," said Mother. "Well, as an adult, I'll do the pouring. Now, drink up your milk while the water heats up." 
All the children watched as Mother poured the water carefully into the dish. Nivedita held the bottle in place by its neck. As Mother poured the water, the balloon started to fill up a little and bulged. Leela was amazed. "How does that happen?" she asked. Maya asked, "What happened?" 
"Can't you see?" asked Leela. She pressed the balloon to show that it had some air inside when earlier there was none. Nivedita said, "If you take a large bottle then it blows up even more. Of course, never as much as if you were blowing it yourself." 
"Where did the air come from?" asked Leela. As Nivedita turned and looked at Mother, she smiled. "This is your experiment; you tell them," she said. Nivedita answered, "When you put the hot water in the dish, the heat is transferred to the bottle. The air inside the bottle gets hot. Now hot air rises and it fills up the balloon since it cannot escape." 
Maya asked, "Where did the air come from?" Nivedita said, "Oh, there's air all around us. This bottle was not empty; it was full ... of air." Maya looked confused. "So an empty bottle is actually full?" As Leela and Nivedita laughed, Maya looked comforted as Mother said, "She's right, you know. It's English that's funny, not Science!"