English: Illustration of Quotient space. The boundary (in blue) of a 2-disk when identified (glued together) to a single point, gives the topological 2-sphere. In general, ${\displaystyle S^{n}=D^{n}/\partial D^{n}}$. More precisely, ${\displaystyle S^{n}}$ is the adjunction space ${\displaystyle x\cup _{f}D^{n}}$, where ${\displaystyle x}$ is a single point, and with ${\displaystyle f:\partial D^{n}\rightarrow x}$ such that ${\displaystyle f(p)=x,\forall p\in \partial D^{n}}$.