The glass, and everything you need to perform A Glass of Water, described in The Hidden Art.
Fill the glass with water and place the lid on top. Press down on the lid to expel a small amount of air from the glass. Then turn the glass upside down while keeping one finger over the small hole.
Remove your finger while keeping the glass inverted. The lower air pressure inside the glass prevents the water from escaping and keeps the lid in place.
The inverted glass is secured to a magnet attached to the included hook. The hook is fastened to the back of your trousers or belt, underneath your jacket.