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  "body": "\n## Summary\n\nAn adjunction space is obtained by gluing a space $Y$ onto a space $X$.\nThe gluing uses a continuous map $f$ from a subspace $A$ of $Y$ into $X$.\nEach point $a$ of $A$ is identified with its image $f(a)$, and the result carries the [[quotient topology|0NQ1H2X495]].\nAttaching cells to build CW complexes and the mapping cylinder are special cases.\n\n## Explanations\n\n### L1 human\n\nThink of $X$ as a base and $Y$ as a piece you want to stick onto it.\nA part $A$ of $Y$ is the glue strip, and the map $f$ says where on $X$ each glue point lands.\nAfter gluing, nothing else changes: the rest of $Y$ stays attached only through $A$.\nFor example, glue a closed disc onto a circle $X$ by sending the boundary circle of the disc onto $X$ by the identity.\nThe result is again a closed disc.\nIf instead the boundary wraps twice around $X$, the result is the real projective plane.\n",
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