ContourPlot[ x^2-y^2,{x,-2,2},{y,-2,2}]

S=Plot3D[y*x^2-x y^2+20Sin[ x y],{x,-2,2},{y,-2,2},PlotRange->All,ColorFunction->Hue,PlotPoints->100,PlotStyle->Thickness[1.2]]

S=Animate[ContourPlot3D[y x^2+z y^2+x z^2==c,{x,-2,2},{y,-2,2},{z,-2,2}],{c,-4,4}];
Export["oliver.mp4",S,"MP4"]

ParametricPlot3D[ {Sin[v] Cos[u],Sin[v] Sin[u],Cos[v]},{v,0,Pi/2},{u,0,Pi},ColorFunction->Hue,Boxed->False,Axes->False,PlotStyle->Thickness[0.3]]

S=ParametricPlot3D[ {(2+Sin[v]) Cos[u],(2+Sin[v])Sin[u],Cos[v]},{v,0,2Pi},{u,0,Pi},Boxed->False,Axes->False,PlotPoints->100,PlotStyle->Thickness[0.2]]
Export["s.stl",S,"STL"]

Import["s.stl"]

r[t_]:={x[t],y[t],z[t]};
s[t_]:=Sqrt[r'[t].r'[t]];
T[t_]:=r'[t]/s[t];
A=Sqrt[T'[t].T'[t]]/s[t];
w[t_]:=Cross[r'[t],r''[t]];
B=Sqrt[w[t].w[t]]/s[t]^3;
FullSimplify[A^2==B^2]