Hydrophobic means repelling or failing to mix with water. The word comes from Greek roots meaning “water-fearing,” and it describes materials that water can't stick to or soak into.

You see hydrophobic surfaces in action when rain beads up on a freshly waxed car instead of spreading out in a wet sheet. The wax creates a hydrophobic coating that makes water form perfect little droplets that roll right off. Duck feathers are naturally hydrophobic, which is why ducks can swim all day without getting waterlogged. Water just slides off their backs, keeping the warm, fluffy down underneath dry.

Scientists and engineers use hydrophobic materials to make waterproof jackets, water-resistant phones, and non-stick cooking pans. Some fabrics are treated with hydrophobic chemicals so rain runs off instead of soaking through.

The opposite of hydrophobic is hydrophilic, which means “water-loving.” Hydrophilic materials absorb water easily, like a sponge or cotton towel. A paper towel is hydrophilic because it soaks up spills, while a plastic bag is hydrophobic because water beads up on its surface. Understanding which materials are hydrophobic and which are hydrophilic helps us choose the right tool for each job.