"Water isn't just H₂O."
Kana stared at the glass of water.
Milia smiled. "Most special molecule on Earth."
"Why?" Toma asked.
Rei answered. "Because it forms hydrogen bonding networks."
"Hydrogen bonds again," Kana opened her notebook.
"But this decides everything," Milia explained. "Water molecule's O is partially negative, H partially positive."
"Electronegativity difference," Toma recalled.
"Yes. Then neighboring water molecule's H and O attract."
Rei drew a diagram. "One water molecule can form up to four hydrogen bonds."
"Four?"
"Oxygen's two lone electron pairs and two hydrogens."
Kana understood. "That's why ice has hexagonal crystals?"
"Correct. Regular network forms."
Milia continued. "But in liquid water, bonds constantly break and form."
"Dynamic structure," Rei supplemented. "Called clusters."
Toma asked. "So what's special?"
"First, high specific heat," Milia answered.
"Specific heat?"
"Heat needed to raise temperature. Water is higher than other liquids."
Rei explained. "Because energy needed to break hydrogen bonds."
"That's why oceans don't warm easily?" Kana confirmed.
"Yes. Stabilizes Earth's temperature."
Toma added. "Also used for body temperature regulation."
"When sweating, evaporation heat lowers body temperature," Milia nodded.
"Evaporation heat?"
"Energy needed to change from liquid to gas. Also energy to break hydrogen bonds."
Kana murmured. "Water is an energy buffer."
"Yes. Functions as cushioning."
Rei mentioned another property. "Surface tension is also high."
"Why water drops are round?" Toma guessed.
"Correct. Water molecules strongly attract each other, minimizing surface area."
Milia gave an example. "Insects can walk on water thanks to surface tension."
"Capillary action too," Rei continued. "Water rises in narrow tubes."
"Why?"
"Surface tension and adhesion to walls. Mechanism for plants to draw up water."
Kana was impressed. "To treetops, without pumps?"
"Tens of meters. Just by hydrogen bonding force."
Toma asked. "Hydrophobic interaction relates to water?"
"Very much," Milia answered. "When oil enters water, hydrogen bonding network is disturbed."
"So it dislikes it?"
"Yes. Water molecules choose to attract each other. Oil is pushed out."
Rei supplemented. "Protein folding is also driven by hydrophobic interactions."
"Proteins?"
"Hydrophobic amino acids inside, hydrophilic outside. Optimizing interaction with water."
Kana understood. "Water determines protein shape."
"Cell membranes too," Milia said. "Bilayer is an arrangement avoiding water."
Toma summarized. "The hidden city built by water molecules?"
Rei answered. "Hydrogen bonding network. Constantly reorganizing dynamic structure."
"Invisible, but exists."
"Life's foundation," Milia said quietly. "Without water, we wouldn't exist."
Kana lifted the glass. "Inside here, there's a city."
"Trillions of water molecules constantly interacting."
"Invisible world."
"But the most important world."
The four gazed at the water. Transparent, appearing empty. But there's complex structure woven by countless bonds. The city built by hydrogen bonds still supports life.