a container cover prevents spillage of contents . in one embodiment , the container cover includes a flexible body having a first portion that can be stretched over a mouth of a container . the container cover also includes a second portion providing a periphery around said first portion that can contract against the sides of a container on which the container cover is placed to hold the cover in position . the first portion when stretched over the mouth of the container has an extension forming an area of slack for the container cover to allow said container cover to expand as a result of increased pressure within the container . the container cover further includes the first portion also including an aperture to receive a tube for withdrawing contents from said container . the first portion of the container cover includes a venting system . the venting system is provided by a valve .