We are going to explore a higher-dimension version of the Poincaré conjecture. In dimension three, this corresponds to the only Millennium Prize problem that is solved as of today. Roughly, the conjecture tells us that, if a manifold is homotopy equivalent to a sphere of its dimension, it is a sphere. We are going to discuss the history of this conjecture, and sketch a proof of the higher-dimension version via the $h$-cobordism theorem, due to Smale (1960). We are also going to introduce handle decompositions and the so-called Whitney trick, due to Whitney, which helps us tidy up handles. Prepare to see a lot of great achievements and of course, a lot of pictures!