Rain streamed down the window glass.
"The water droplets are connecting," Kana put her hand on the window.
Milia smiled. "Thanks to hydrogen bonding."
"Hydrogen bonding?"
Rei opened a notebook. "Water molecules, H₂O. Oxygen has high electronegativity, so it attracts electrons."
"Electronegativity..." Kana repeated.
"The power of an atom to pull electrons. Oxygen is strong," Milia supplemented.
Rei drew a diagram. "So the oxygen side becomes slightly negative, the hydrogen side slightly positive."
"Polarity, right?"
"Yes. Because it's a polar molecule, hydrogen attracts the oxygen of neighboring molecules. That's hydrogen bonding."
Kana gazed at the window glass. "These water droplets are also connected molecule to molecule?"
"Trillions of them," Milia said quietly.
Rei continued. "One hydrogen bond is weak. But in large quantities, they become powerful."
"So that's why water is liquid at room temperature?" Kana understood.
"Molecular weight 18, yet boiling point is 100°C. Methane, molecular weight 16, boils at -162°C."
Milia added. "If there were no hydrogen bonds, all Earth's water would be gas."
Kana imagined. "A world without oceans..."
"And no life," Rei said quietly.
"Why?"
"Because water is the stage for biochemical reactions."
Milia showed a glass of water. "Countless molecules are dissolved in this."
"Water is a universal solvent," Rei explained. "Being polar, it dissolves ions and polar molecules."
"Add NaCl?" Kana asked.
"It separates into Na⁺ and Cl⁻. Water molecules surround and hydrate them."
Milia added. "Oxygen side stabilizes Na⁺, hydrogen side stabilizes Cl⁻."
"So that's why salt dissolves."
Rei gave another example. "But oil doesn't dissolve."
"Why?"
"Oil is nonpolar. It doesn't mix with water."
Milia said. "Hydrophobic interactions. These create life's structures."
"What do you mean?" Kana showed interest.
"Consider cell membranes. Lipid tails are hydrophobic, heads are hydrophilic."
Rei drew a diagram. "So they form a bilayer. Hydrophobic parts inside, hydrophilic parts outside."
"They gather as if escaping from water?"
"Yes. Entropically favorable," Milia acknowledged.
Kana lifted the glass. "Surface tension is also from hydrogen bonding?"
"Exactly. Surface water molecules are pulled by molecules below and beside them."
"So water droplets become round."
Rei nodded. "They try to minimize surface area."
Milia pointed outside the window. "See the water droplets on the leaves?"
"They're rolling," Kana said.
"Water repellency. The leaf surface is covered with hydrophobic wax."
Rei continued. "Water becomes spherical, reducing contact area."
Kana pondered. "Water isn't just an ordinary liquid."
"For life, the most important molecule," Milia said quietly.
"Why?"
Rei organized. "Solvent, reactant, structure formation, temperature regulation... involved in everything."
"Temperature regulation?"
"High specific heat. Hard to warm up, hard to cool down."
Milia added. "Because energy is needed to break hydrogen bonds."
"So oceans moderate temperature changes," Kana understood.
"Body temperature also stabilizes," Rei said.
Kana touched the window glass. "Each water droplet has a story."
"A molecular story," Milia smiled.
Rei closed the notebook. "Hydrogen bonding made life possible."
Rain continued falling quietly.
"Thank you, water," Kana murmured.
Milia and Rei laughed.
"Thanking molecules is so like Kana."
The three gazed out the window. Hidden in the raindrops were life's secrets.