TITLE: Ideal of ideal needs not to be an ideal
QUESTION [10 upvotes]: Suppose I is an ideal of a ring R and J is an ideal of I, is there any counter example showing J need not to be an ideal of R? The hint given in the book is to consider polynomial ring with coefficient from a field, thanks

REPLY [4 votes]: Consider $R=\mathbb Q[x]$, and $I=xR$ be the most obvious ideal of $R$.
Note that we can define $J$ as a subset of $I$ to be an ideal of $I$ if $J$ is a subgroup of $(I,+)$ and $IJ\subseteq J$.  Find a $J$ that is a super-set of $x^2R$ but does not contain all of $I=xR$.