Float fun

D. Leela and class-mates, Chennai

"What are you doing, Maya?" shouted Leela from the other end of the swimming pool. "Come and play ball."
"I'm trying to go under water," said Maya. "But the more I dive under the water, the faster I float up. I've dropped my glasses and I can't pick them up from the bottom."
"Let me get it for you," said Leela, swimming across towards Maya.  "Where did you drop it?"
"There it is. I can see it under water but I just can't get to it. The more I kick my legs the faster I bounce back to the top!"
As she was talking, Leela smoothly dived into the water. In a short time, she came up, glasses in hand, and held it out to Maya.
"How did you do it?" asked Maya, half enviously. I just can't seem to sink into the water!" 
"Easy," said Leela. She kicked her feet at an angle, and went down again!
"What's the point in saying, 'Easy'," grumbled Maya. "Saying easy doesn't make it easy!"
Mother was listening and laughed at this. "Don't you think it's nicer to be able to float on water than worry about not being able to go under?  Look at that young girl who's been struggling to learn to swim!"
Leela said, "I remember when I first learned to swim, I just couldn't keep my head out of water. I would come away from the pool with a stomach-full of water each time! Maya, you didn't have so much difficulty at all. In fact you learned to swim faster than I did. I'm sure you'll learn to dive under water soon."
Maya said, "If our bodies are lighter than water, then we should always float. And if our bodies were heavier, then we should always sink.  Leela keeps floating when she's swimming up and down the pool. But when she wants to dive to the bottom of the pool, she's actually sinking in some sense. How does she do it?"
Leela said, "I know why we float. We learned it in school. It's not the weight, but the density. Things float when they have a lower density than the fluid they displace."
Mother said, "That's right." If an object is put in water, it displaces some of that water. If the weight of that displaced volume of water is more than the weight of the object, it will float.  You'll float if your average body density is less than that of water."
"But why is it that some people swim easily and some take so long to learn?" asked Leela.
"Well, that just depends on ability", said Mother. "But some children are more chubby than others. Fat is less dense than water (cream always floats on top of milk). So fatter children float more easily. Some thin or bony children can be marginally denser than water. Then the only way they can keep swimming is to make sure they have plenty of air in their lungs. That will make their average density less than that of water and they will float."
"Why do we have to worry about averages?" asked Leela.
"It's easier to understand if you think of a ship. A steel ship appears to be very heavy and the density of steel is also very high (8 times more than that of water). So you would expect it to sink. But if you make the shape of the ship so that it holds a lot of air, then the average density of the ship is the average of steel and air. Since air is very light, the average density becomes less than that of water and so the ship floats."
"That's why a damaged ship sinks, is it?" asked Maya. "When water enters, the air is removed and so the density goes up. Is that it?"
"Right," said Mother. 
Maya was still thinking. "Amma, so if Leela's density is less than water, how can she 'sink' when she dives under water? I don't understand."
Leela said, "I got it. When I am going under, I actually breathe out. So there's less air in my lungs, and I go down like a damaged ship!"
"That's the trick, is it?" said Maya, exhaling deeply as she kicked and smoothly dived beneath the calm water.