A tamper evident cap adapted to close an opening on a vessel includes a main body having an outer circumference, a pull tab extending from the main body, and a flange, forms an annulus around the main body and connected to the main body. The flange includes an outer circumference that is larger than the outer circumference of the main body. It is essential to the present invention that the flange is made of a very thin layer of material. In addition, the flange includes at least a pair of perforation lines. Further, the tamper evident cap includes a bump in the pull-tab for enabling a person grasping the pull-tab, to obtain a firmer hold thereon.