In a small lake at the edge of the forest lived a little golden fish named Sparkle. She was a cheerful and very curious fish, whose eyes shone like two little stars in the night sky. Sparkle loved warm summer days most of all, when she could bask near the surface and observe the world above the water.
"Today is such a beautiful sunny day," she thought to herself as she floated just below the surface of the lake. The sun's rays passed through the water and gently caressed her scales.
Sparkle loved to play with her friends – little frog Hopper, who could tell stories about the world above the surface, and Scaly, an older carp who knew all the secrets of the lake.
"Hey, Sparkle!" called Hopper, who was sitting on a water lily leaf. "Why do you always stay near the surface? There are so many interesting things down below!"
Sparkle swam over to the lily pad. "But it's so cold down there! Whenever I swim deeper, I feel the water getting colder."
"That's true," nodded Scaly, who had just swum up from the depths. "The water is colder down below, especially in summer."
Sparkle tilted her little head and her scales glittered in the sun. "But why is that? Why is the water cold below and warm above? The sun shines on the whole lake."
Scaly smiled. "That's a good question, little Sparkle. If you want to know the answer, you must become a researcher and figure it out yourself."
"A researcher?" Sparkle widened her big eyes. "And how do I become a researcher?"
"You need to observe, measure, and do experiments," explained Hopper. "My grandfather was a scientist-frog and he always said that there's an answer to every question, you just have to look for it correctly."
Sparkle spun excitedly in the water. "Then I'll be a researcher! I'll find out why the water is cold down below!"
The next morning Sparkle woke up with great determination. She borrowed a small thermometer from Hopper, which the frog had found by the shore when people were having a picnic there.
"First I need to take measurements," Sparkle said to herself.
She started at the surface. She held the thermometer and carefully watched as the red line settled. "At the surface we have... twenty-two degrees," she announced to her imaginary diary.
Then she swam a bit deeper. "One meter below the surface we have... twenty degrees."
The deeper she swam, the colder the water became. At the bottom of the lake, the thermometer showed only fifteen degrees.
"That's interesting," thought Sparkle. "The deeper, the colder. But why?"
She decided she needed help and called her friends for a meeting.
"I have a mystery," she explained to them. "I measured that the water is much colder at the bottom than up at the surface. But I don't understand why."
Snail Slimy, who had also crawled to the meeting, said: "Maybe it's because the sun heats the upper layer of water first."
"That makes sense," nodded Sparkle, "but why doesn't the warm water mix with the cold water then?"
Just then old Catfish swam by, the oldest inhabitant of the lake. He had long whiskers and eyes full of wisdom.
"I heard you're doing research, little Sparkle," he said in a deep voice.
"Yes, I'm trying to find out why the water is colder down below," answered Sparkle.
Catfish smiled. "You know, that's one of nature's most interesting phenomena. Water has a special property – when it heats up, it becomes lighter."
"Lighter?" Sparkle tilted her head in confusion.
"Yes, lighter," nodded Catfish. "When water heats up in the sun, its molecules start moving faster and take up more space. So warm water is thinner and lighter than cold water."
"And since it's lighter, then..." began Sparkle.
"Exactly!" rejoiced Catfish. "Since it's lighter, it stays up top, while the heavier cold water sinks down. That's why we have warm water at the surface and cold water at the bottom."
Sparkle thought about this. "So it's like a balloon filled with hot air? That also rises up because hot air is lighter?"
"Exactly! You're a smart fish. It's called thermal stratification of water."
"Thermal stratification," repeated Sparkle. "That's amazing! So our lake has three floors like some kind of water house!"
"Exactly so," laughed Catfish. "In winter, however, it's different. Then everything mixes up."
Sparkle was confused. "Why does it mix up in winter?"
Catfish shook his head. "Because water has yet another special property. When it cools down, it becomes denser and sinks down, but only until four degrees Celsius. Then, when it cools further toward zero, it starts becoming lighter again."
"That's quite a mystery!" exclaimed Sparkle.
"Thanks to this we can survive in winter," explained Catfish. "If water kept getting denser as it cooled, ice would form from the bottom of the lake and everything would freeze. This way ice forms on the surface and protects the water beneath it, which stays liquid."
Sparkle was amazed. It would never have occurred to her that ordinary water could have such miraculous properties. "So that's why we can survive at the bottom of the lake in winter?"
"Yes," nodded Catfish. "And that's also why in summer we have pleasantly cool water in the depths, where we can hide from the heat."
Sparkle decided she had to complete her research with an experiment. Together with Hopper and Scaly, they put water in a transparent bottle, added a few grains of sand so they could see the flow, and placed it in the sun. After a while they saw how the warm water stayed on top and the cold water below. Two layers formed, just like in the lake.
"Hooray! Our experiment confirmed it!" rejoiced Sparkle.
In the evening she organized a big presentation for all the inhabitants of the lake. On a large rock by the shore she explained her research and drew pictures in the fine sand at the bottom.
"And so I discovered why the water is colder at the bottom," she concluded her presentation. "It's because warm water is lighter and stays on top, while cold, heavier water sinks down."
All the fish, frogs, and snails clapped with their fins, little legs, and tentacles. They were amazed at what an important discovery their little friend had made.
"And what did you learn from this research?" asked old Catfish with a smile.
Sparkle thought about it. "I learned that even small fish can be great scientists. And that nature has its reasons for everything it does. Thanks to water's special properties, we can all live in the lake all year round – in summer and winter."
From that day on, Sparkle became not only the most curious, but also the wisest fish in the lake. And when other fish asked her why she was so enthusiastic about science, she always answered with a smile: "Because every drop of water hides infinite mysteries that are waiting for someone to solve them."