托福听力背景知识：SurfaceTension

Surface Tension

　　condition existing at the free surface of a liquid, resembling the properties of an elastic skin under tension. The tension is the result of intermolecular forces exerting an unbalanced inward pull on the individual surface molecules; this is reflected in the considerable curvature at those edges where the liquid is in contact with the wall of a vessel. More specifically, the tension is the force per unit length of any straight line on the liquid surface that the surface layers on the opposite sides of the line exert upon each other. See Cohesion.

 
　　The tendency of any liquid surface is to become as small as possible as a result of this tension, as in the case of mercury, which forms an almost round ball when a small quantity is placed on a horizontal surface. The near-perfect spherical shape of a soap bubble, which is the result of the distribution of tension on the thin film of soap, is another example of this force; surface tension alone can support a needle placed horizontally on a water surface. See also Capillary Action.

Squid

　　The squid breathes through gills, and may emit a cloud of inky material from its ink sac when in danger. The circulatory and nervous systems are highly developed.

 

　　The eye of the squid is remarkably similar to that of man—an example of convergent evolution, as there is no common ancestor. Some deep-sea forms have luminescent organs.