TITLE: Is $\{\emptyset\}$ a subset of $\{\{\emptyset\}\}$?
QUESTION [26 upvotes]: $\{\emptyset\}$ is a set containing the empty set. Is $\{\emptyset\}$ a subset of $\{\{\emptyset\}\}$?
My hypothesis is yes by looking at the form of "the superset $\{\{\emptyset\}\}$" which contains "the subset $\{\emptyset\}$".

REPLY [18 votes]: For such abstract questions, it is important that you stick to the definitions of the involved notions.
By definition, $A$ is a subset of $B$ if every element contained in $A$ is also contained in $B$.
Now we look at $A = \{\emptyset\}$ and $B = \{\{\emptyset\}\}$.
$A$ has exactly one element, namely $\emptyset$.
This is not an element of $B$, since the only element of $B$ is $\{\emptyset\}$ and $\emptyset \neq \{\emptyset\}$ (see below).
So $A$ is not a subset of $B$.
Why $\emptyset \neq \{\emptyset\}$?
By definition, two sets are equal if they contain the same elements. However, $\emptyset$ is the empty set without any element, but $\{\emptyset\}$ is a $1$-element set with the element $\emptyset$.