The Dehn's lemma states that if a continuous map from a 2-disk to a 3-manifold $f\colon D^2\to M$ for which $f^{-1}f\partial D^2=D^2$ then there exists an embedding $g\colon D^2\to M$ such that the restrictions $ g\vert\partial$ and $ f\vert\partial$ are equal.