What is a knot?

Knots are all around us on shoelaces, on climbing or sailing ropes, and some tiny ones on complicated molecules and DNAs. These examples of knots mostly appear on strings with loose ends. However in Mathematics we like to think that the ends are glued together so that it cannot be undone. So knots are tangled loops on a rubber string that can be stretched, twisted but never torn or glued. The driving question of knot theory is to tell knots apart from each other or tell if a knot can be completely untangled into a circle. In this talk through some examples I will introduce the basic tools to deal with such questions called knot invariants.