Let S={1,2,3}. Then ∅⊆ S. Why is this true? ∅⊆{∅}. Why is this true?
The statement that $\displaystyle A\subseteq B$ means that $\displaystyle \text{if }x\in A\text{ then }x\in B~.$
So for any set $\displaystyle X$ it is true that $\displaystyle \emptyset\subseteq X$