Sailor's expertise is demonstrated in their ability to tie knots. Knots are an essential skill for sailors, as they are used to secure sails and rigging, as well as to join two ropes together. Knots can also be used to create a loop or a noose, which can be used to attach items to a boat or to secure a line. Knowing how to tie knots is an important part of a sailor's job, and it requires practice and experience to become an expert.