"Water is mysterious."
Kana stared at a glass.
Rei was interested. "How so?"
"It's so ordinary that it doesn't feel special. But thinking about it, it's full of mysteries."
Milia smiled. "Good observation."
"What mysteries?" Rei asked.
Kana thought. "Like, why does it boil at 100 degrees?"
"It's such a small molecule with molecular weight 18."
Rei nodded. "Sharp question."
"Methane has molecular weight 16, boiling point minus 161 degrees."
"Water is clearly abnormal."
Milia began explaining. "Hydrogen bonding."
"Look at H2O structure." She assembled a molecular model.
"Oxygen in center, two hydrogens."
"Bent shape."
Rei supplemented. "HOH angle, about 104.5 degrees."
"This asymmetry creates polarity."
Kana wrote in her notebook. "Polarity = charge distribution."
"Oxygen side negative, hydrogen side positive."
Milia continued. "A neighboring water molecule's hydrogen is attracted to another water molecule's oxygen."
"This is hydrogen bonding."
Rei drew a diagram. "One water molecule can bond with up to four water molecules."
"Three-dimensional network."
Kana was surprised. "Like a web."
"Yes. Though liquid, it's quite structured."
Milia explained. "Breaking these bonds requires energy."
"That's why the boiling point is high."
Rei added. "One hydrogen bond is about 20 kJ/mol."
"Weaker than covalent bonds but stronger than van der Waals forces."
Kana asked. "Ice has an even more rigid structure?"
"Correct," Milia acknowledged. "Crystal structure, all hydrogen bonds in optimal arrangement."
"So it has lower density than water."
"Wait, ice is lighter?"
Rei explained. "Because the hydrogen bond network expands."
"More gaps."
"That's why ice floats on water."
Kana was moved. "That's really important, isn't it?"
"Why?"
"When a lake freezes, it freezes from the top. The water below stays liquid."
Milia nodded. "Fish can survive."
"If ice sank, lakes would freeze from the bottom and everything would die."
Rei became philosophical. "Life's coincidences depend on water's properties."
Kana raised another question. "Why are water droplets round?"
"Surface tension," Milia answered.
"Surface water molecules can only hydrogen bond on one side."
"They're pulled inward."
Rei supplemented. "Force to minimize surface area."
"Sphere is the shape with minimum surface area."
Kana proposed an experiment. "I want to verify it."
Milia took out a needle. "Can float it on water surface."
Carefully placing it, the needle floated.
"But it's metal!"
"Surface tension supports it," Rei explained.
"Water's membrane has elasticity."
Kana was impressed. "Water really is special."
Milia continued. "Excellent as a solvent too."
"Being polar, it dissolves ions and polar molecules."
Rei gave examples. "Salt, sugar, amino acids..."
"Most substances necessary for life dissolve in water."
"That's why cells are full of water."
Kana summarized. "The hydrogen bond network makes everything possible."
Milia nodded. "But easily broken, quickly rebuilt."
"Balancing liquid fluidity and structural stability."
Rei added. "Dynamic equilibrium."
"Bonds constantly breaking and forming."
Kana drank the water from the glass. "Are hundreds of millions of hydrogen bonds changing right now?"
"Can't calculate exactly, but a vast number," Milia answered.
Rei said finally. "Water is the solvent of life, a structural material, and a reactant."
"The stage for all biochemical reactions."
Kana looked at the window. Rain began falling.
"Earth, the water planet."
The three quietly watched the rain. The network of water molecules expands endlessly.