Real liquids are compressible. The compressibility of liquids is described by modulus of elasticity (or by coefficient of volume compressibility) analogous to solids. A value of modulus of elasticity at liquids depends on many factors, above all on volume of free gases in liquids and further on pressure and temperature of liquids. and Skutečné kapaliny jsou stlačitelné. Stlačitelnost kapalin je určena modulem pružnosti (resp. součinitelem objemové stlačitelnosti) podobně jako u pevných látek. Velikost modulu pružnosti u kapalin závisí na mnoha faktorech, zejména na obsahu volných plynů v kapalinách a dále na tlaku a teplotě kapalin.
Ornithologists in Finland recorded the autumn migration of the Red Admiral butterfly, Vanessa atalanta (L.). In the best year, 1998, 1240 migrants were counted from a bird tower in September. That is, half a million butterflies migrated over a 100-km front. The butterflies were flying above forests riding on cool northerly winds. Radar indicated that a large proportion migrated at high elevations outside the visible range. These records help to resolve an "evolutionary puzzle" of why migrant butterflies and moths travel to northern latitudes when their offspring have limited possibilities of returning south. Until now, the only butterfly for which a high-elevation return migration on northerly winds is known is the American Monarch Danaus plexippus (L.).