A steel blade placed gently on the surface of water floats on it. If the same blade is kept well inside the water, it sinks. Explain.
Due to the surface tension the surface of the water acts like a stretched membrane. The surface tension along the edges of the blade is more than the weight of the blade thus it floats. Now the surface tension is a surface phenomenon, so when the same blade is kept well inside the water there is no surface tension to counter its weight. There is only the weight of the blade and buoyancy force on it. Since the density of the blade is more than the water, the weight is greater than the buoyancy force, thus it sinks.