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  "body": "\n## Summary\n\nA monad on a category is an endofunctor $T$ with a unit $\\eta \\colon \\mathrm{Id} \\Rightarrow T$ and a multiplication $\\mu \\colon T T \\Rightarrow T$ that satisfy associativity and unit laws.\nIt is the categorical form of a monoid, with composition of [[functors|28NQ0KRHD3]] in place of multiplication.\nEvery pair of [[adjoint functors|TZHF6ZM92S]] gives a monad, and every monad comes from an adjunction.\nMonads describe algebraic structure: groups, monoids, rings and many other algebraic categories are the algebras of a monad on sets.\n\n## Explanations\n\n### L1 human\n\nThink of $T(X)$ as the set of formal expressions built from elements of $X$.\nThe unit turns an element into the simplest expression, the element itself.\nThe multiplication flattens an expression made of expressions into a single expression.\nFor example, let $T(X)$ be the set of finite lists of elements of $X$.\nThe unit sends $x$ to the one-element list $(x)$, and the multiplication concatenates a list of lists into one list.\n",
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