Picard Existance Existence Theorem:
Consider $z'(t)=f(t,z)$. Define $\mathbf{F}y:=y_0+\int_{0}^tf(x,y(t))dx$. \mathbf{F}y:=y_0+\int_0^t f(x,y(t))dx$. It sure would be nice if $\mathbf{F}$ has had a fixed point, so use Banach fixed point theorem to show that it has a fixed point. That would require $f$ to be a contraction, so sprinkle a hint of Lipschitz on $f$.

