Why can a water strider (and even a steel pin) rest on water without sinking?
The water surface behaves like a stretched elastic film because of surface tension — molecules at the surface are pulled inward, so the surface resists being pierced.
The water surface behaves like a stretched elastic film because of surface tension — molecules at the surface are pulled inward, so the surface resists being pierced. As long as the insect's weight is small enough for the surface to support (spread over its water-repellent legs), the film curves and holds it up. Add a drop of detergent and surface tension collapses, so it sinks.
Key point
Surface tension makes the liquid surface act like a stretched membrane.
Common mistake
thinking the insect floats by buoyancy like a boat.
Memory tip
surface tension holds it on top; soap kills the tension and it sinks.
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