The Assassin’s Teapot is something I first heard about during my childhood, that is, a long time ago. I’ve always been fascinated with its concept, as it is very simple — and, in the wrong hands, likely deadly.
As quoted from Jason Kottke post about it:
The teapot in question has two separate chambers for holding liquid, and the flow out of the pot from each chamber can be controlled by covering or uncovering small holes located on the handle. So, as the legend goes, a would-be assassin could pour themselves a perfectly fine drink from one chamber and then pour a poisoned drink to their prey from the other chamber, just by discreetly covering and uncovering the proper holes with their fingers.
Jason also mentions a YouTube video from Steve Mould. Steve does science videos, and an amazing job describing the way the teapot works.