Ever-filling Glass - Locking
Performer shows a transparent glass full of liquid. He drinks from it., till the glass is almost empty. The glass is placed behind his back, or under his coat for a moment, and brought in view is seen to been full of liquid once again. This can be repeated as often as desired.
You can use this special glass in any liquid effect, to show a glass full of liquid or empty, as desired. A strong magicial effect of filling a glass itself visibly with liquid.