The Assassin’s Teapot

The assassin’s teapot is a cleverly designed container that can pour from different reservoirs depending on how it’s held. Steve Mould digs into the physics in this video, and he builds a transparent cutaway version of the pot to show exactly how it works. This design uses two separate reservoirs, each with two holes — one in the spout and one concealed near the pot’s handle. By covering this breather hole, the server blocks air from flowing into the teapot, which also keeps the liquid inside from flowing out.

What holds the liquid in? Air pressure, with an assist from surface tension. Atmospheric pressure is enough to hold the fluid inside the pot, provided air has no separate way in. To get in through the spout, air would have to push into the pot at the same time as water coming out. Surface tension prevents that, though, because the spout is too narrow. The same physics keeps water inside a larger bottle with a wire mesh over its mouth. The mesh’s tiny holes are smaller than the capillary length of water, which is the length scale at which surface tension and gravity balance one another. As long as the spout and holes are smaller than that length, surface tension will keep the liquid from deforming enough to get out. (Video and image credit: S. Mould)