"...He gives an example of the sand castles children make on the beach with the sand. We adults smile when the children make these castles, spending a lot of time and effort making them, eventually to be washed away by the waves hitting the beach sooner or later.
We adults smile when we see the sand castle being washed away. But we don't realize that we do the same thing all our lives also. We spend all our lives building big houses (castles), and building our wealth, careers, etc. only to lose them all in a few years upon death, if not earlier.
Did you get the hint?"