> restart:
> r:=0.5:
> x:=r+r*cos(t):
> y:=r*sin(t):
> z1:=sqrt(1-x^2-y^2):
> z2:=-z1:
> z:=(1-s)*z1+s*z2:
> plot3d ([x,y,z],s=0..1,t=0..2*Pi,color=red,scaling=constrained);