What is a union? Marital, setwise, or otherwise, they're all types of colimits. Enter the hermeneutic circle hand-in-hand with Yoneda, learn a family recipe for triangles, and discover crossword puzzles are colimits, too! We'll be extremely concrete in discussions of unions, coequalizers, colimits, the Yoneda Lemma, Simplicial objects, sheaves, and ultimately topoi.
Colimits Aren't Stupid -- A Geometric Realization