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  "body": "\n## Summary\n\nThe join $X * Y$ of two topological spaces is the space of all segments that run from a point of $X$ to a point of $Y$.\nFormally it is $X \\times Y \\times [0, 1]$ with the end $t = 0$ collapsed onto $X$ and the end $t = 1$ collapsed onto $Y$.\nJoins generalise cones and suspensions, and the join of two spheres is again a sphere: $S^m * S^n \\cong S^{m+n+1}$.\n\n## Explanations\n\n### L1 human\n\nPlace $X$ and $Y$ in space far from each other and draw a straight segment from every point of $X$ to every point of $Y$.\nThe union of all these segments is the join.\nFor example, the join of two points is a segment, and the join of a point with a circle is a cone.\nThe join of two disjoint segments in general position in $\\R^3$ is a solid tetrahedron.\n",
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