> restart:

> x:=cos(t):

> y:=sin(t):

> z1:=x:

> z2:=-x:

> z:=(1-s)*z1+s*z2:

> a:=plot3d([x,y,z],s=0..1,t=0..2*Pi,scaling=constrained,color=red):

> b:=plot3d([z,y,x],s=0..1,t=0..2*Pi,scaling=constrained,color=green):

> with (plots):

> display(a,b);