Martin really likes tea. In fact he likes it so much that he made his own magic teapot. The first time he pours from it he gets black tea, the second time he pours milk, and finally he pours tea with milk. Upon closer inspection the teapot has no buttons or electronics, so how does it work?
The teapot has four spouts, two for milk and two for tea (connecting to separate internal chambers). They’re arranged with a tea spot at 0 degrees, a milk spout at 120 degrees, and both a milk spout and a tea spout at 240 degrees. Martin pours in a different direction each time.
Or: Martin empties out and refills the teapot each time.
Or: The teapot has valves which are controlled by levers, not buttons.
Or: The teapot has a clockwork mechanism which opens different valves each time it’s tilted.