This is interesting to me but I have a question on the definition of a fluid that is presented here. It says it is a medium that cannot resist any stress applied to it —- is that not what surface tension does? Anyone smarter than me got an explanation?
Your question actually suggests that you are smarter than me, but anyway: the surface tension of an ideal fluid is indeed zero.
Real fluids are just approximations of the ideal. :) Their fundamental flaw is not being truly continuous, but rather consisting of sticky molecules. Surface tension is caused by the imbalance of forces acting on molecules not surrounded by other molecules on all sides:
