Definition
Surface tension is the force that makes the molecules at a liquid's surface pull hard on one another, so the liquid shrinks its surface area as much as it can. The top layer acts like a stretched, springy skin, and that skin holds the liquid in a round shape.
How a Water Strider Walks on Water
Stroll along a calm pond and you may spot a water strider with long, thin legs gliding across the surface like a skater, without sinking. Even a steel paper clip or a needle, much heavier than water, can float if you set it down gently.
Many people assume this is buoyancy, the lift that floats things lighter than water. But the real reason a water strider's feet don't break through is that the water's surface holds firm like a tightly stretched rubber sheet, spreading out the weight pressing down on it.
The same thing explains why morning dew on a blade of grass, or a raindrop on a window, beads up instead of spreading flat. A liquid tends to make its exposed surface as small as possible. And for a given volume, the solid shape with the smallest surface area is a perfect ball.
The Lonely Pull on Surface Molecules
The root cause is that a water molecule deep inside gets pulled differently from one at the surface. A molecule in the middle of the water is surrounded on every side by other water molecules. Neighbors tug on it equally from above, below, left, and right, so the pulls cancel out and it stays in a calm, stable state.
A molecule right at the surface, where water meets air, is in a very different spot. Below it and beside it, water molecules are packed tightly and pull on it. Above it, in the air, there are almost no water molecules.
So a surface molecule gets no pull from above, and it feels a strong pull toward the inside of the liquid. Every surface molecule huddles inward and clings to its neighbors, and the whole surface tightens without a gap. That tightness is surface tension.
A Little More Precisely
More scientifically, surface tension is the energy needed to stretch a liquid's surface by one unit of area. To pull molecules that are being tugged inward out to the surface and enlarge it, you must spend outside energy to break the strong attractions between molecules.
Water has an unusually high surface tension compared with most liquids. That is because water molecules bond to one another through their oxygen and hydrogen atoms, using a special attraction called a hydrogen bond. Liquids such as alcohol or oil can't hold a round bead for long and quickly flatten out, because their molecules cling to each other less strongly than water's do.
What happens if you drop a little soap or dish detergent into water? Surfactant molecules in the detergent rush to the surface and take the place of water molecules there. The surface tension drops in an instant, the round beads of water spread out limply, and the water soaks into every corner of cloth or dishes.
Common misconceptions
A water strider floats because its body is lighter than water.
A water strider stays up not because it is light, but because its feet rest on the springy skin of the water's surface, formed by surface tension. Add a drop of detergent to weaken the surface tension, and the water strider sinks.
Where you meet it
Morning dew on a blade of grass keeps a round, bead-like shape instead of flattening out.
A glass filled to the brim doesn't spill right away. The water bulges above the rim.
A steel paper clip or needle set gently on water floats instead of sinking.
In one sentence
It is the force with which surface water molecules pull one another inward, shrinking the surface area as much as possible.