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Hi I can Plot the result of an NDSolve (toy example), but I cannot ParametricPlot it.

sol = NDSolve[{s'[t] == -s[t], i'[t] == -s[t], s[0] == 1, 
    i[0] == .1}, {s[t], i[t]}, {t, 0, 2}] ;

Plot[{  s[t] /. sol, i[t] /. sol}, {t, 0, 2}]
ParametricPlot[{  s[t] /. sol, i[t] /. sol}, {t, 0, 2}]
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Finally, just adding Evaluate does it

ParametricPlot[Evaluate[{  s[t], i[t]} /. sol], {t, 0, 2}]

Still, I would like to understand why that is required with ParametricPlot but not with Plot

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    $\begingroup$ I believe you should execute the argument of ParametricPlot in each to see the difference. I think you’ll find that the code in the question does not have the form of xy coordinates. (In the docs, they sometimes use First[sol] instead of sol for this reason.) $\endgroup$
    – Michael E2
    Commented Jun 14, 2021 at 4:31
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    $\begingroup$ ParametricPlot[{s[t], i[t]} /. sol, {t, 0, 2}] or ParametricPlot[{s[t] /. sol[[1]], i[t] /. sol[[1]]}, {t, 0, 2}] $\endgroup$
    – cvgmt
    Commented Jun 14, 2021 at 8:38

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