The usual argument is this, roughly phrased due to your "simple language" request.
If there is NOT another Rational Number between any two given Rational Numbers, why can I construct one?!
Since we can construct one, we must believe there is one.
View topic - finite rational number • Art of Problem Solving
please explain this post in a very simple language i try to understand each and every answer but could not understand .how can finite rational number can exist in an interval however small.is there is any gap between two rational number.